Derivatives (Forward Contracts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional amount	$ 206.3
Fair value investments	3,208.3	$ 2,547.3	$ 1,543.5
Forward Contracts
Derivative [Line Items]
Realized and unrealized gains (losses)	(23.8)	(1.2)
Other Investments
Derivative [Line Items]
Net amount recognized in other log term investments	(3.7)	(1.2)
GBP
Derivative [Line Items]
Notional amount	152.0
Barclays Bank PLC | A | Forward Contracts
Derivative [Line Items]
Notional amount	206.3
Fair value investments	$ (3.7)	(1.2)
Barclays Bank PLC | A- | Forward Contracts
Derivative [Line Items]
Notional amount		184.6
Carrying value		$ (1.2)