Derivative Instruments (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements		$ 44,000			$ 44,000			$ 40,600
Derivative liabilities subject to master netting agreements		20,100			20,100			19,600
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	[1]	(2,146)		$ (4,770)	(6,109)		$ (13,951)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value		46,384			46,384			42,931
Liability derivatives - fair value		(22,398)			(22,398)			(20,464)
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(2,146)		(4,770)	(6,109)		(13,951)
Not Designated as Hedging Instrument [Member] | Futures contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,276,189			1,276,189			1,452,497
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(240)		(5,310)	4,790		2,410
Not Designated as Hedging Instrument [Member] | Futures contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value		556	[3]		556	[3]		3,371
Not Designated as Hedging Instrument [Member] | Futures contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(3,425)			(3,425)			(1,542)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,072,743			1,072,743			686,941
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(1,692)		(272)	(7,616)		(12,038)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	18,183			18,183			4,478
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(7,732)			(7,732)			(4,381)
Not Designated as Hedging Instrument [Member] | TBAs
Derivative [Line Items]
Net derivatives - notional value	[2],[4]	14,480			14,480			27,066
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		0		86	(97)		21
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale
Derivative [Line Items]
Asset derivatives - fair value		14,761	[4]		14,761	[4]		27,184
Liability derivatives - fair value	[4]	0			0			0
Not Designated as Hedging Instrument [Member] | Other
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	1,924,299			1,924,299			1,457,345
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(214)		$ 726	(3,186)		$ (4,344)
Not Designated as Hedging Instrument [Member] | Other | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	12,884			12,884			7,898
Not Designated as Hedging Instrument [Member] | Other | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	$ (11,241)			$ (11,241)			$ (14,541)

[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.’