Derivative Instruments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements		$ 105,500		$ 105,500		$ 80,400
Derivative liabilities subject to master netting agreements		52,700		52,700		18,900
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	[1]	42,893	$ (17,556)	142,730	$ (23,665)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value		106,608		106,608		82,185
Liability derivatives - fair value		(53,740)		(53,740)		(20,664)
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		42,893	(17,556)	142,730	(23,665)
Not Designated as Hedging Instrument [Member] | Futures contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	3,987,260		3,987,260		3,153,518
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		46,194	(15,399)	140,503	(10,609)
Not Designated as Hedging Instrument [Member] | Futures contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	15,148		15,148		51,800
Not Designated as Hedging Instrument [Member] | Futures contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(20,515)		(20,515)		(2,115)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	965,348		965,348		1,008,907
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		2,044	(5,458)	(17,030)	(13,074)
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	5,316		5,316		8,147
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	(8,616)		(8,616)		(7,796)
Not Designated as Hedging Instrument [Member] | TBAs
Derivative [Line Items]
Net derivatives - notional value	[2],[4]	53,229		53,229		8,132
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		269	(3)	507	(100)
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale
Derivative [Line Items]
Asset derivatives - fair value	[4]	54,961		54,961		8,292
Liability derivatives - fair value	[4]	0		0		0
Not Designated as Hedging Instrument [Member] | Other
Derivative [Line Items]
Net derivatives - notional value	[2],[3]	3,983,466		3,983,466		2,213,981
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments		(5,614)	$ 3,304	18,750	$ 118
Not Designated as Hedging Instrument [Member] | Other | Other assets
Derivative [Line Items]
Asset derivatives - fair value	[3]	31,183		31,183		13,946
Not Designated as Hedging Instrument [Member] | Other | Other liabilities
Derivative [Line Items]
Liability derivatives - fair value	[3]	$ (24,609)		$ (24,609)		$ (10,753)

[1] See note 8 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.’