Derivative Instruments (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements		$ 97,800	$ 80,400
Derivative liabilities subject to master netting agreements		37,800	18,900
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	[1]	119,741	15,636	$ (7,356)
Not Designated as Hedging Instrument
Derivative [Line Items]
Asset derivatives - fair value		103,956	82,185
Liability derivatives - fair value		(39,750)	(20,664)
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		119,741	15,636	(7,356)
Not Designated as Hedging Instrument | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		114,123	48,443	9,318
Not Designated as Hedging Instrument | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	[2]	10,065	51,800
Liability derivatives - fair value	[2]	(13,722)	(2,115)
Net derivatives - notional value	[2],[3]	4,104,559	3,153,518
Not Designated as Hedging Instrument | Foreign currency forward contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(9,499)	(21,770)	(14,495)
Not Designated as Hedging Instrument | Foreign currency forward contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	[2]	5,352	8,147
Liability derivatives - fair value	[2]	(5,327)	(7,796)
Net derivatives - notional value	[2],[3]	686,878	1,008,907
Not Designated as Hedging Instrument | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		463	(133)	9
Not Designated as Hedging Instrument | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	[4]	55,010	8,292
Liability derivatives - fair value	[4]	0	0
Net derivatives - notional value	[3],[4]	53,229	8,132
Not Designated as Hedging Instrument | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		14,654	(10,904)	$ (2,188)
Not Designated as Hedging Instrument | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	[2]	33,529	13,946
Liability derivatives - fair value	[2]	(20,701)	(10,753)
Net derivatives - notional value	[2],[3]	$ 4,356,300	$ 2,213,981

[1] See Note 10 for information on the Company’s derivative instruments.

[2] The fair value of asset derivatives are included in ‘other assets’ and the fair value of liability derivatives are included in ‘other liabilities.’ Such amounts include risk in force on GSE credit-risk sharing transactions that are accounted for as derivatives.

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

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