Summary of Significant Accounting Policies - Schedule of Effects of Reclassifications out of Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains	$ 1,512			$ 3,211			$ 14,555	$ 17,043
Credit losses on available-for-sale securities	178						(155)
Earnings before income taxes	51,509			38,947			87,024	169,516
Income tax expense	(9,122)			(6,623)			(13,738)	(31,252)
Net earnings	42,387	$ 92,166	$ (61,267)	32,324	$ 40,467	$ 65,473	73,286	138,264
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out of Accumulated Other Comprehensive Income
Net realized gains	738			1,126			3,356	2,415
Credit losses on available-for-sale securities	353						(604)
Earnings before income taxes	1,091			1,126			2,752	2,415
Income tax expense	(229)			(236)			(578)	(507)
Net earnings	$ 862			$ 890			$ 2,174	$ 1,908