Summary of Significant Accounting Policies - Schedule of Unrealized Gains and Losses on Available-for-Sale Securities Reclassified out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net investment gains (losses)									$ 21,071	$ 14,088	$ 8,423
Earnings before Federal income taxes									112,068	165,156	141,507
Federal income taxes									19,756	33,540	24,749
Net earnings	$ 35,135	$ 10,825	$ 48,413	$ (2,062)	$ 37,733	$ 19,989	$ 33,696	$ 40,198	92,312	131,616	116,758
Accumulated Other Comprehensive Income | Amount Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net investment gains (losses)									5,677	2,787	3,441
Net OTTI losses recognized in earnings									0	(7,847)	0
Earnings before Federal income taxes									5,677	(5,060)	3,441
Federal income taxes									1,192	(1,063)	723
Net earnings									$ 4,485	$ (3,997)	$ 2,718