Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value investments	$ 2,532.5	$ 2,610.8	$ 3,208.3	$ 2,547.3
Forward Contracts
Derivative [Line Items]
Notional amount			206.3
Fair value investments			$ (3.7)
Realized and unrealized gains (losses)	$ (3.5)	$ 3.0