Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value investments	$ 2,633.8	$ 2,275.4	$ 2,633.8	$ 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)	$ (10.2)	$ (3.5)	$ (13.0)