Fair Value Accounting - Assets and liabilities measured at fair value on recurring basis (Details) - Fair value measurements, recurring - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 2 [Member]
Assets:
Total assets	$ 0.1	$ 2.9
Liabilities:
Total liabilities	12.9	0.0
Fair Value, Inputs, Level 2 [Member] | Other assets
Assets:
Equity instruments	0.1	1.3
Fair Value, Inputs, Level 2 [Member] | Other assets | Interest rate hedges
Assets:
Derivative asset	0.0	1.6
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	124.9	113.1
Restricted cash	171.6	195.2
Total assets	296.5	308.3
Level 3
Assets:
Total assets	0.0
Wholly Owned Subsidiaries [Member] | Fair Value, Inputs, Level 2 [Member] | Accrued liabilities | Interest rate hedges
Liabilities:
Liability	$ 12.9	$ 0.0