Fair Value Accounting (Details) (Fair value measurements, recurring, USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Level 1
Assets:
Cash equivalents	$ 159.7		$ 230.6
Restricted cash	231.4		260.7
Total assets	391.1		491.3
Liabilities:
Total liabilities	0		0
Level 1 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	0	[1]	0	[1]
Level 1 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	0	[1]	0	[1]
Level 2
Assets:
Cash equivalents	0		0
Restricted cash	0		0
Total assets	0		0
Liabilities:
Total liabilities	20.1		23.8
Level 2 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	18.0	[1]	21.7	[1]
Level 2 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	2.1	[1]	2.1	[1]
Level 3
Assets:
Cash equivalents	0		0
Restricted cash	0		0
Total assets	0		0
Liabilities:
Total liabilities	0		0
Level 3 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	0	[1]	0	[1]
Level 3 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	0	[1]	0	[1]
Total
Assets:
Cash equivalents	159.7		230.6
Restricted cash	231.4		260.7
Total assets	391.1		491.3
Liabilities:
Total liabilities	20.1		23.8
Total | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	18.0	[1]	21.7	[1]
Total | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Interest rate hedges	$ 2.1	[1]	$ 2.1	[1]

[1] Included in accrued liabilities on the consolidated balance sheet.