Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount	$ 299.9
Forward Contracts
Derivative [Line Items]
Notional amount	299.9
Derivative, fair value, net	(4.2)
Realized and unrealized gains (losses)	(3.0)
Other Investments [Member]
Derivative [Line Items]
Derivative Liability	(4.2)	$ (1.2)
United Kingdom, Pounds
Derivative [Line Items]
Notional amount	200.0
Euro Member Countries, Euro
Derivative [Line Items]
Notional amount	50.0
Barclays [Member] | Standard & Poor's, A- Rating [Member] | Forward Contracts
Derivative [Line Items]
Notional amount	247.2
Derivative, fair value, net	(3.2)
JP Morgan | Standard & Poor's, A- Rating [Member] | Forward Contracts
Derivative [Line Items]
Notional amount	52.7
Derivative, fair value, net	$ (1.0)