Fair Value Disclosure - Balance Sheet Grouping (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recourse fixed rate debt	$ 3,999.5	$ 3,971.2
Recourse floating rate debt	424.3	426.0
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recourse fixed rate debt	3,952.9	4,089.1
Recourse floating rate debt	$ 423.0	$ 428.7