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, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 584,535	$ 612,029
Derivative Assets	13,491	17,079
Derivative Liabilities	19,259	14,822
Gains (Losses)	(38,421)	12,350
Exchange-traded futures
Derivative [Line Items]
Notional Value	171,112	218,657
Derivative Assets	939	1,594
Derivative Liabilities	871	2,534
Gains (Losses)	(10,840)	3,515
Currency forwards
Derivative [Line Items]
Notional Value	60,809	87,019
Derivative Assets	8,545	7,647
Derivative Liabilities	8,633	7,582
Gains (Losses)	738	379
Interest rate swaps
Derivative [Line Items]
Notional Value	92,756	112,658
Derivative Assets	1,746	1,649
Derivative Liabilities	2,254	1,959
Gains (Losses)	(616)	(125)
Credit default swaps
Derivative [Line Items]
Notional Value	168,303	94,657
Derivative Assets	2,151	2,888
Derivative Liabilities	5,611	2,685
Gains (Losses)	(6,413)	335
Total return swaps
Derivative [Line Items]
Notional Value	91,201	99,038
Derivative Assets	110	3,301
Derivative Liabilities	1,764	62
Gains (Losses)	(21,164)	$ 8,246
Option swaps
Derivative [Line Items]
Notional Value	354
Derivative Assets	0
Derivative Liabilities	126
Gains (Losses)	$ (126)