Condensed Financial Information of Registrant (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Activities
Net cash provided by operating activities	$ 306.8	$ 243.7	$ 267.0
Investing Activities
Proceeds from sale-leaseback		79.0	45.7
Portfolio proceeds and other	154.1	84.3	67.9
Purchases of leased-in assets	(61.1)	(5.3)	(10.7)
Net cash used in investing activities	(458.0)	(417.7)	(308.2)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	312.8	344.2	480.8
Net (decrease) increase in debt with original maturities of 90 days or less	(85.0)	46.8	(55.3)
Proceeds from issuances of debt (original maturities longer than 90 days)	790.3	573.8	636.1
Stock repurchases			(55.1)
Employee exercises of stock options	6.6	2.5
Dividends	(56.0)	(53.5)	(53.6)
Net cash provided by (used in) financing activities	323.2	211.0	(19.5)
Net increase (decrease) in Cash and Cash Equivalents during the period	169.9	36.8	(60.5)
Cash and Cash Equivalents at beginning of period	78.5	41.7	102.2
Cash and Cash Equivalents at end of period	248.4	78.5	41.7
Parent Company [Member]
Operating Activities
Net cash provided by operating activities	95.0	92.8	232.6
Investing Activities
Capital additions	(343.1)	(417.1)	(340.1)
Proceeds from sale-leaseback		79.0	45.7
Portfolio proceeds and other	143.0	65.4	66.5
Purchases of leased-in assets	(61.1)	(5.3)	(10.7)
Capital contributions to subsidiaries, net	(25.7)		(30.8)
Net cash used in investing activities	(286.9)	(278.0)	(269.4)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(195.9)	(286.3)	(435.4)
Net (decrease) increase in debt with original maturities of 90 days or less	(89.1)	58.6	(78.1)
Proceeds from issuances of debt (original maturities longer than 90 days)	701.0	493.5	636.1
Stock repurchases			(55.1)
Employee exercises of stock options	6.6	2.5
Dividends	(56.0)	(53.5)	(53.6)
Other	(15.2)	(8.8)	(13.0)
Net cash provided by (used in) financing activities	351.4	206.0	0.9
Net increase (decrease) in Cash and Cash Equivalents during the period	159.5	20.8	(35.9)
Cash and Cash Equivalents at beginning of period	48.9	28.1	64.0
Cash and Cash Equivalents at end of period	208.4	48.9	28.1
Total Distributions from Subsidiaries		$ 57.3