Fair Value Accounting - Assets and liabilities Measured at fair value on recurring basis (Details) - Fair value measurements, recurring - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Cash equivalents	$ 456.9	$ 113.1
Restricted cash	142.2	195.2
Total assets	603.1	311.2
Other assets
Assets:
Equity instruments	1.3	1.3
Other assets | Interest rate hedge
Assets:
Derivative asset	2.7	1.6
Level 1
Assets:
Cash equivalents	456.9	113.1
Restricted cash	142.2	195.2
Total assets	599.1	308.3
Level 1 | Other assets
Assets:
Equity instruments	0.0	0.0
Level 1 | Other assets | Interest rate hedge
Assets:
Derivative asset	0.0	0.0
Level 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Total assets	4.0	2.9
Level 2 | Other assets
Assets:
Equity instruments	1.3	1.3
Level 2 | Other assets | Interest rate hedge
Assets:
Derivative asset	2.7	1.6
Level 3
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Total assets	0.0	0.0
Level 3 | Other assets
Assets:
Equity instruments	0.0	0.0
Level 3 | Other assets | Interest rate hedge
Assets:
Derivative asset	$ 0.0	$ 0.0