Condensed Financial Information of Registrant - Statements of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities
Net cash provided by operating activities	$ 370.2	$ 306.8	$ 243.7
Investing Activities
Proceeds from sale-leasebacks	104.9	0	79.0
Portfolio proceeds and other	288.9	154.1	84.3
Purchases of leased-in assets	(1.3)	(61.1)	(5.3)
Net cash used in investing activities	(343.6)	(458.0)	(417.7)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(671.2)	(312.8)	(344.2)
Net increase (decrease) in debt with original maturities of 90 days or less	243.3	(85.0)	46.8
Proceeds from issuances of debt (original maturities longer than 90 days)	445.2	790.3	573.8
Dividends	(58.8)	(56.0)	(53.5)
Net cash (used in) provided by financing activities	(39.9)	323.2	211.0
Net (decrease) increase in cash and cash equivalents during the period	(14.2)	169.9	36.8
Cash and Cash Equivalents at beginning of period	248.4	78.5	41.7
Cash and Cash Equivalents at end of period	234.2	248.4	78.5
Parent Company [Member]
Operating Activities
Net cash provided by operating activities	269.6	95.0	92.8
Investing Activities
Capital additions	(444.6)	(343.1)	(417.1)
Proceeds from sale-leasebacks	104.9	0	79.0
Portfolio proceeds and other	136.6	143.0	65.4
Purchases of leased-in assets	(1.4)	(61.1)	(5.3)
Capital contributions to affiliates, net	0	(25.7)	0
Net cash used in investing activities	(204.5)	(286.9)	(278.0)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(603.7)	(195.9)	(286.3)
Net increase (decrease) in debt with original maturities of 90 days or less	185.0	(89.1)	58.6
Proceeds from issuances of debt (original maturities longer than 90 days)	381.4	701.0	493.5
Dividends	(57.7)	(56.0)	(53.5)
Other	5.0	(8.6)	(6.3)
Net cash (used in) provided by financing activities	(90.0)	351.4	206.0
Net (decrease) increase in cash and cash equivalents during the period	(24.9)	159.5	20.8
Cash and Cash Equivalents at beginning of period	208.4	48.9	28.1
Cash and Cash Equivalents at end of period	183.5	208.4	48.9
Total Distributions from Affiliates	$ 101.0	$ 0	$ 57.3