Leases.  Our operating leases principally involve office space and office equipment. Rental expense for operating leases, which is recognized on a straight-line basis over the lease term, was $22.3 million, $22.0 million and $20.5 million for the twelve months ended December 31, 2012, 2011 and 2010, respectively. Our headquarters building ground lease has purchase options exercisable beginning in 2019, renewal options exercisable in 2048 and escalation clauses that began in 2009. Expected future minimum payment obligations for non-cancelable operating leases exceeding one year are as follows as of December 31, 2012: