Fair Value Accounting - Assets and liabilities Measured at fair value on recurring basis (Details) - Fair value measurements, recurring - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Restricted cash	$ 199.9	$ 113.1
Restricted cash	138.5	195.2
Total assets	341.8	311.2
Other assets
Assets:
Equity instruments(1)	0.4	1.3
Other assets | Interest rate hedge
Assets:
Derivative Asset	3.0	1.6
Level 1
Assets:
Restricted cash	199.9	113.1
Restricted cash	138.5	195.2
Total assets	338.4	308.3
Level 1 | Other assets
Assets:
Equity instruments(1)	0.0	0.0
Level 1 | Other assets | Interest rate hedge
Assets:
Derivative Asset	0.0	0.0
Level 2
Assets:
Restricted cash	0.0	0.0
Restricted cash	0.0	0.0
Total assets	3.4	2.9
Level 2 | Other assets
Assets:
Equity instruments(1)	0.4	1.3
Level 2 | Other assets | Interest rate hedge
Assets:
Derivative Asset	3.0	1.6
Level 3
Assets:
Restricted cash	0.0	0.0
Restricted cash	0.0	0.0
Total assets	0.0	0.0
Level 3 | Other assets
Assets:
Equity instruments(1)	0.0	0.0
Level 3 | Other assets | Interest rate hedge
Assets:
Derivative Asset	$ 0.0	$ 0.0