Derivative Instruments - Notional Value, Fair Value and Gains and Losses Recognized in Investment Gains (Losses) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value	$ 489,985	$ 584,535
Derivative Assets	21,741	13,491
Derivative Liabilities	30,537	19,259
Gains (Losses)	(35,589)	(38,421)
Exchange-traded futures
Derivative [Line Items]
Notional Value	142,886	171,112
Derivative Assets	118	939
Derivative Liabilities	1,834	871
Gains (Losses)	(15,743)	(10,840)
Currency forwards
Derivative [Line Items]
Notional Value	63,906	60,809
Derivative Assets	8,576	8,545
Derivative Liabilities	8,451	8,633
Gains (Losses)	(1,779)	738
Interest rate swaps
Derivative [Line Items]
Notional Value	60,997	92,756
Derivative Assets	2,043	1,746
Derivative Liabilities	2,955	2,254
Gains (Losses)	(347)	(616)
Credit default swaps
Derivative [Line Items]
Notional Value	167,649	168,303
Derivative Assets	10,910	2,151
Derivative Liabilities	13,304	5,611
Gains (Losses)	(104)	(6,413)
Total return swaps
Derivative [Line Items]
Notional Value	52,061	91,201
Derivative Assets	94	110
Derivative Liabilities	1,847	1,764
Gains (Losses)	(15,242)	(21,164)
Option swaps
Derivative [Line Items]
Notional Value	2,486	354
Derivative Assets	0	0
Derivative Liabilities	2,146	126
Gains (Losses)	$ (2,374)	$ (126)