Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements	$ 165,400,000		$ 26,900,000
Derivative liabilities subject to master netting agreements	72,300,000		7,800,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value	437,993,000		26,908,000
Liability derivatives - fair value	(296,365,000)		(7,752,000)
Net derivatives - fair value	141,628,000		19,156,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	(378,000)	(4,669,000)
Not Designated as Hedging Instrument [Member] | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	(829,000)	(2,021,000)
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	638,000		52,000
Liability derivatives - fair value	(175,000)		(52,000)
Net derivatives - fair value	463,000		0
Net derivatives - notional value	311,532,000		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	(725,000)	(1,881,000)
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	2,598,000		2,809,000
Liability derivatives - fair value	(4,617,000)		(2,678,000)
Net derivatives - fair value	(2,019,000)		131,000
Net derivatives - notional value	542,136,000		396,468,000
Not Designated as Hedging Instrument [Member] | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	541,000	323,000
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	432,730,000		23,599,000
Liability derivatives - fair value	(290,526,000)		(4,346,000)
Net derivatives - fair value	142,204,000		19,253,000
Net derivatives - notional value	684,500,000		26,000,000
Not Designated as Hedging Instrument [Member] | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	635,000	(1,090,000)
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	2,027,000		448,000
Liability derivatives - fair value	(1,047,000)		(676,000)
Net derivatives - fair value	980,000		(228,000)
Net derivatives - notional value	$ 287,716,000		$ 50,341,000