Annual report pursuant to Section 13 and 15(d)

Financial Information by Geographic Area (Detail)

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Financial Information by Geographic Area (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue $ 558.10 $ 543.90 $ 535.80 $ 522.70 $ 509.70 $ 490.40 $ 487.10 $ 472.60 $ 2,160.50 $ 1,959.80 $ 1,859.50
Percentage of operating revenues                 100.00% 100.00% 100.00%
Long-lived assets 3,981.4       3,060.5       3,981.4 3,060.5  
Percentage of consolidated long-lived assets                 100.00% 100.00%  
UNITED STATES
                     
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 1,649.40 1,440.90 1,352.20
Percentage of operating revenues                 76.00% 74.00% 73.00%
Long-lived assets 3,453.6       2,542.4       3,453.6 2,542.4  
Percentage of consolidated long-lived assets                 87.00% 83.00%  
Canada
                     
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 153.90 151.30 138.40
Percentage of operating revenues                 7.00% 8.00% 7.00%
Long-lived assets 63.9       70.7       63.9 70.7  
Percentage of consolidated long-lived assets                 1.00% 2.00%  
UNITED KINGDOM
                     
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 133.50 124.10 104.70
Percentage of operating revenues                 6.00% 6.00% 6.00%
Long-lived assets 122.3       121.5       122.3 121.5  
Percentage of consolidated long-lived assets                 3.00% 4.00%  
BRAZIL
                     
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 0 35.40 84.10
Percentage of operating revenues                 0.00% 2.00% 4.00%
Long-lived assets 70.8       77.2       70.8 77.2  
Percentage of consolidated long-lived assets                 2.00% 3.00%  
Other Countries
                     
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 223.70 208.10 180.10
Percentage of operating revenues                 11.00% 10.00% 10.00%
Long-lived assets $ 270.8       $ 248.7       $ 270.8 $ 248.7  
Percentage of consolidated long-lived assets                 7.00% 8.00%