Derivative Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements	$ 25,300,000		$ 28,000,000
Derivative liabilities subject to master netting agreements	5,400,000		14,600,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	43,934,000	[1]	20,912,000	[1]	(1,326,000)	[1]
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value	26,468,000		39,859,000
Liability derivatives - fair value	(5,397,000)		(26,472,000)
Net derivatives - fair value	21,071,000		13,387,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	43,934,000		20,912,000		(1,326,000)
Not Designated as Hedging Instrument [Member] | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	30,444,000		10,742,000		(3,307,000)
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	2,156,000		461,000
Liability derivatives - fair value	(1,907,000)		(110,000)
Net derivatives - fair value	249,000		351,000
Net derivatives - notional value	2,549,027,000	[2]	475,967,000	[2]
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	13,430,000		9,762,000		(214,000)
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	10,511,000		5,023,000
Liability derivatives - fair value	(1,145,000)		(3,090,000)
Net derivatives - fair value	9,366,000		1,933,000
Net derivatives - notional value	397,106,000	[2]	330,746,000	[2]
Not Designated as Hedging Instrument [Member] | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	(342,000)		(1,623,000)		4,413,000
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	10,592,000		33,455,000
Liability derivatives - fair value	0		(21,731,000)
Net derivatives - fair value	10,592,000		11,724,000
Net derivatives - notional value	10,056,000	[2]	56,160,000	[2]
Not Designated as Hedging Instrument [Member] | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	402,000		2,031,000		(2,218,000)
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	3,209,000		920,000
Liability derivatives - fair value	(2,345,000)		(1,541,000)
Net derivatives - fair value	864,000		(621,000)
Net derivatives - notional value	$ 735,684,000	[2]	$ 347,916,000	[2]

[1] See Note 11 for information on the Company’s derivative instruments.

[2] Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.