Derivatives Forward Contracts (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Forward Contracts
Derivative [Line Items]
Notional amount	$ 302.0	$ 302.0
Realized and unrealized gains (losses)	(1.7)	1.7
Unrealized gain (loss) on derivatives	8.5	11.3
Derivative, fair value, net	(12.5)	(12.5)
Other Investments
Derivative [Line Items]
Derivative liability	12.5	12.5	$ 1.2
United States of America, Dollars
Derivative [Line Items]
Notional amount	302.0	302.0
Derivative, fair value, net	57.0	57.0
United Kingdom, Pounds
Derivative [Line Items]
Notional amount	150.0	150.0
Euro Member Countries, Euro
Derivative [Line Items]
Notional amount	104.0	104.0
Derivative, fair value, net	50.0	50.0
Barclays \| Standard & Poor's, A- Rating \| Forward Contracts
Derivative [Line Items]
Notional amount	187.3	187.3
Derivative, fair value, net	(8.2)	(8.2)
JP Morgan \| United States of America, Dollars
Derivative [Line Items]
Notional amount	52.7	52.7
JP Morgan \| Standard & Poor's, A plus \| Forward Contracts
Derivative [Line Items]
Notional amount	114.7	114.7
Derivative, fair value, net	$ (4.3)	$ (4.3)

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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