Significant Accounting Policies (Gain on Asset Dispositions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Significant Acquisitions and Disposals [Line Items]
Disposition gains	$ 42.7	$ 40.4	$ 23.8
Residual sharing income	22.6	5.2	7.6
Non-remarketing disposition gains	19.2	27.0	18.0
Asset impairments	(5.0)	(6.8)	(8.3)
Net Gain on Asset Dispositions	$ 79.5	$ 65.8	$ 41.1