Schedule I - Condensed Financial Information of Registrant - Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities
Net cash provided by operating activities	$ 534.3	$ 449.2	$ 400.7
Investing Activities
Purchases of leased-in assets	(118.4)	(150.5)	(61.4)
Proceeds from sale-leasebacks	0.0	0.0	90.7
Portfolio proceeds and other	482.2	264.0	385.3
Net cash used in investing activities	(307.6)	(878.5)	(403.2)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(726.3)	(819.8)	(602.8)
Net increase (decrease) in debt with original maturities of 90 days or less	(64.5)	50.0	(251.3)
Proceeds from issuances of debt (original maturities longer than 90 days)	748.8	1,223.0	1,132.2
Stock repurchases	(125.4)	(124.6)	(68.6)
Dividends	(68.2)	(62.0)	(60.5)
Net cash (used in) provided by financing activities	(229.0)	262.2	149.1
Net (decrease) increase in cash and cash equivalents during the period	(7.5)	(169.8)	145.5
Cash and Cash Equivalents at beginning of period	209.9	379.7	234.2
Cash and Cash Equivalents at end of period	202.4	209.9	379.7
Parent Company [Member]
Operating Activities
Net cash provided by operating activities	412.5	219.5	104.3
Investing Activities
Capital additions	602.9	748.1	513.5
Purchases of leased-in assets	(118.4)	(150.5)	(61.4)
Proceeds from sale-leasebacks	0.0	0.0	90.7
Portfolio proceeds and other	208.7	169.5	320.7
Net cash used in investing activities	(512.6)	(729.1)	(163.5)
Financing Activities
Repayments of debt (original maturities longer than 90 days)	(350.0)	(692.2)	(483.8)
Net increase (decrease) in debt with original maturities of 90 days or less	(69.0)	69.0	(185.0)
Proceeds from issuances of debt (original maturities longer than 90 days)	695.7	1,188.7	967.0
Stock repurchases	(125.4)	(124.6)	(68.6)
Dividends	(68.2)	(62.0)	(60.5)
Other	2.3	(1.6)	1.2
Net cash (used in) provided by financing activities	85.4	377.3	170.3
Net (decrease) increase in cash and cash equivalents during the period	(14.7)	(132.3)	111.1
Cash and Cash Equivalents at beginning of period	162.3	294.6	183.5
Cash and Cash Equivalents at end of period	$ 147.6	$ 162.3	$ 294.6