Condensed Financial Information of Registrant - Statements of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Activities
Net cash provided by operating activities	$ 400.7	$ 370.2	$ 306.8
Investing Activities
Purchases of leased-in assets	(61.4)	(1.3)	(61.1)
Proceeds from sale-leasebacks	90.7	104.9	0
Portfolio proceeds and other	385.3	288.9	154.1
Net cash used in investing activities	(403.2)	(343.6)	(458.0)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(602.8)	(671.2)	(312.8)
Net increase (decrease) in debt with original maturities of 90 days or less	(251.3)	243.3	(85.0)
Proceeds from issuances of debt (original maturities longer than 90 days)	1,132.2	445.2	790.3
Stock repurchased	68.6	0	0
Dividends	(60.5)	(58.8)	(56.0)
Net cash (used in) provided by financing activities	149.1	39.9	323.2
Net (decrease) increase in cash and cash equivalents during the period	145.5	(14.2)	169.9
Cash and Cash Equivalents at beginning of period	234.2	248.4	78.5
Cash and Cash Equivalents at end of period	379.7	234.2	248.4
Parent Company [Member]
Operating Activities
Net cash provided by operating activities	104.3	269.6	95.0
Investing Activities
Capital additions	513.5	444.6	343.1
Purchases of leased-in assets	(61.4)	(1.4)	(61.1)
Proceeds from sale-leasebacks	90.7	104.9	0
Portfolio proceeds and other	320.7	136.6	143.0
Capital contributions to affiliates, net	0	0	25.7
Net cash used in investing activities	(163.5)	(204.5)	(286.9)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(483.8)	(603.7)	(195.9)
Net increase (decrease) in debt with original maturities of 90 days or less	(185.0)	185.0	(89.1)
Proceeds from issuances of debt (original maturities longer than 90 days)	967.0	381.4	701.0
Stock repurchased	(68.6)	0	0
Dividends	(60.5)	(57.7)	(56.0)
Other	1.2	5.0	(8.6)
Net cash (used in) provided by financing activities	170.3	(90.0)	351.4
Net (decrease) increase in cash and cash equivalents during the period	111.1	(24.9)	159.5
Cash and Cash Equivalents at beginning of period	183.5	208.4	48.9
Cash and Cash Equivalents at end of period	294.6	183.5	208.4
Total Distributions (to) from Affiliates	$ (313.8)	$ 101.0	$ 0