Derivative Instruments (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements	$ 223,500,000				$ 223,500,000				$ 26,900,000
Derivative liabilities subject to master netting agreements	76,800,000				76,800,000				7,800,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	15,646,000	[1]	277,000	[1]	15,268,000	[1]	(4,392,000)	[1]
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value	452,824,000				452,824,000				26,908,000
Liability derivatives - fair value	(278,328,000)				(278,328,000)				(7,752,000)
Net derivatives - fair value	174,496,000				174,496,000				19,156,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	15,646,000		277,000		15,268,000		(4,392,000)
Not Designated as Hedging Instrument [Member] | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	10,623,000		(2,322,000)		9,794,000		(4,343,000)
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	160,000				160,000				52,000
Liability derivatives - fair value	(54,000)				(54,000)				(52,000)
Net derivatives - fair value	106,000				106,000				0
Net derivatives - notional value	303,286,000				303,286,000				340,400,000
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	7,605,000		938,000		6,880,000		(943,000)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	15,779,000				15,779,000				2,809,000
Liability derivatives - fair value	(7,605,000)				(7,605,000)				(2,678,000)
Net derivatives - fair value	8,174,000				8,174,000				131,000
Net derivatives - notional value	887,815,000				887,815,000				396,468,000
Not Designated as Hedging Instrument [Member] | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	(3,751,000)		1,419,000		(3,210,000)		1,742,000
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	436,215,000				436,215,000				23,599,000
Liability derivatives - fair value	(270,629,000)				(270,629,000)				(4,346,000)
Net derivatives - fair value	165,586,000				165,586,000				19,253,000
Net derivatives - notional value	687,560,000				687,560,000				26,000,000
Not Designated as Hedging Instrument [Member] | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	1,169,000		242,000		1,804,000		(848,000)
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	670,000				670,000				448,000
Liability derivatives - fair value	(40,000)				(40,000)				(676,000)
Net derivatives - fair value	630,000				630,000				(228,000)
Net derivatives - notional value	$ 203,466,000				$ 203,466,000				$ 50,341,000

[1] See Note 9 for information on the Company’s derivative instruments.