Investments - Equity Method LLCs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Equity Method Investment, Summarized Financial Information, Assets	$ 2,805.3	$ 2,393.7	$ 2,618.1
Equity Method Investment, Summarized Financial Information, Liabilities	1,030.7	899.7	828.0
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	130.4	209.3	$ 85.0
Equity method limited liability investments at cost plus cumulative undistributed earnings	187.0	$ 161.0
Outstanding commitments to fund equity method limited liability investments	$ 112.4