Derivatives - Income Statement (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	$ (37,037)	$ 6,004	$ 7,366
Derivates not designated as hedges [Member] | Included in net foreign exchange gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	38,589	(64,183)	27,263
Derivates not designated as hedges [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(75,626)	70,187	(19,897)
Derivates not designated as hedges [Member] | Foreign exchange forward contracts [Member] | Included in net foreign exchange gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	39,399	(59,019)	23,474
Derivates not designated as hedges [Member] | Foreign currency option contracts [Member] | Included in net foreign exchange gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(810)	(5,164)	3,789
Derivates not designated as hedges [Member] | Futures contracts [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(72,146)	78,841	(31,757)
Derivates not designated as hedges [Member] | Credit default swaps (protection purchased) [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(3)	(134)	(907)
Derivates not designated as hedges [Member] | Credit default swaps (assumed risks) [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	0	123	2,016
Derivates not designated as hedges [Member] | Insurance-linked securities [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	230	(707)	4,343
Derivates not designated as hedges [Member] | Total return swaps [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(1,002)	(6,597)	(749)
Derivates not designated as hedges [Member] | Interest rate swaps [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(15,871)	7,469	112
Derivates not designated as hedges [Member] | TBAs [Member] | Included in net realized and unrealized investment gains and losses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	$ 13,166	$ (8,808)	$ 7,045