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, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue $ 509.7 $ 490.4 $ 487.1 $ 472.6 $ 482.0 $ 473.8 $ 460.7 $ 443.0 $ 1,959.8 $ 1,859.5 $ 1,716.0
Percentage of operating revenues                 100.00% 100.00% 100.00%
Long-lived assets 3,056.3       3,004.4       3,056.3 3,004.4  
Percentage of consolidated long-lived assets                 100.00% 100.00%  
UNITED STATES
                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 1,440.9 1,352.2 1,254.6
Percentage of operating revenues                 74.00% 73.00% 73.00%
Long-lived assets 2,538.2       2,535.2       2,538.2 2,535.2  
Percentage of consolidated long-lived assets                 83.00% 84.00%  
Canada
                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 151.3 138.4 122.6
Percentage of operating revenues                 8.00% 7.00% 7.00%
Long-lived assets 77.2       170.9       77.2 170.9  
Percentage of consolidated long-lived assets                 3.00% 6.00%  
UNITED KINGDOM
                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 124.1 104.7 104.9
Percentage of operating revenues                 6.00% 6.00% 6.00%
Long-lived assets 121.5       93.2       121.5 93.2  
Percentage of consolidated long-lived assets                 4.00% 3.00%  
BRAZIL
                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 35.4 84.1 82.3
Percentage of operating revenues                 2.00% 4.00% 5.00%
Long-lived assets 70.7       93.1       70.7 93.1  
Percentage of consolidated long-lived assets                 2.00% 3.00%  
Other Countries
                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Operating revenue                 208.1 180.1 151.6
Percentage of operating revenues                 10.00% 10.00% 9.00%
Long-lived assets $ 248.7       $ 112.0       $ 248.7 $ 112.0  
Percentage of consolidated long-lived assets                 8.00% 4.00%