Derivative Instruments (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements		$ 33,100			$ 33,100			$ 28,400
Derivative liabilities subject to master netting agreements		34,500			34,500			26,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	[1]	(4,770)		$ 20,334	(13,951)		$ 41,066
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value		35,185			35,185			30,001
Liability derivatives - fair value		(35,004)			(35,004)			(26,049)
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(4,770)		20,334	(13,951)		41,066
Not Designated as Hedging Instrument [Member] | Futures contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	2,402,654			2,402,654			1,655,530
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(5,310)		34,871	2,410		61,322
Not Designated as Hedging Instrument [Member] | Futures contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value		4,490	[3]		4,490	[3]		360
Not Designated as Hedging Instrument [Member] | Futures contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(9,988)			(9,988)			(9,398)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,068,430			1,068,430			1,186,386
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(272)		(13,782)	(12,038)		(18,534)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	13,854			13,854			9,354
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(16,659)			(16,659)			(12,941)
Not Designated as Hedging Instrument [Member] | TBAs
Derivative [Line Items]
Net derivatives - notional value	[2],[4]	4,978			4,978			0
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		86		37	21		334
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale
Derivative [Line Items]
Asset derivatives - fair value		4,970	[4]		4,970	[4]		0
Liability derivatives - fair value	[4]	0			0			0
Not Designated as Hedging Instrument [Member] | Other
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,747,025			1,747,025			1,014,863
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		726		$ (792)	(4,344)		$ (2,056)
Not Designated as Hedging Instrument [Member] | Other | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	11,871			11,871			20,287
Not Designated as Hedging Instrument [Member] | Other | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	$ (8,357)			$ (8,357)			$ (3,710)

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

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

[3] The fair value of asset derivatives are included in ‘other assets’ and the fair value of liability derivatives are included in ‘other liabilities.’

[4] The fair value of TBAs are included in ‘fixed maturities available for sale, at fair value.’