Investments - Equity Method LLCs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Equity Method Investment, Summarized Financial Information, Assets	$ 2,618.1	$ 3,801.7
Equity Method Investment, Summarized Financial Information, Liabilities	828.0	879.1
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	85.0	159.6	$ 96.2
Equity method limited liability investments at cost plus cumulative undistributed earnings	175.9	$ 190.6
Outstanding commitments to fund equity method limited liability investments	$ 76.3