Derivative Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 07, 2016
Derivative [Line Items]
Notional amount of derivative	$ 15,264.6	$ 13,128.8	$ 15,264.6	$ 13,128.8
Total Return Swap (TRS)
Derivative [Line Items]
Unutilized portion of facility accounted for as a derivative	625.0	625.0	625.0	625.0
Notional amount of derivative	182.4	587.5	182.4	587.5
Actual adjusted qualifying borrowing outstanding	442.6	474.8	442.6	474.8
Maximum aggregate facility commitment amounts		1,062.3		1,062.3
Liability recorded based on Company's valuation	$ 14.1	11.3	14.1	11.3
Increase (decrease in liability)			$ 2.8	$ (43.6)
Present value of remaining facility fee					$ 280.0
Decrease in liability associated with TRS transaction		37.0
Pretax charge		$ 245.0
Fixed facility fee	2.85%