Fair Value Disclosure Fair Value Disclosure - Other Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recourse fixed rate debt	$ 4,436.1	$ 3,933.4
Recourse floating rate debt	361.4	522.7
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recourse fixed rate debt	4,470.6	3,836.0
Recourse floating rate debt	$ 359.8	$ 515.1