Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ (13,287)	$ 2,193	$ 45,843	$ 30,927	$ 9,703	$ 29,155	$ 30,446	$ 7,244	$ 65,676	$ 76,548	$ (59,319)
Net investment income	(271,609)	(266,926)	(261,244)	(257,105)	(255,483)	(246,711)	(244,712)	(244,894)	(1,056,884)	(991,800)	(956,690)
Other operating expense									347,833	353,954	322,029
Total before tax									(1,194,266)	(1,102,111)	(1,274,611)
Tax									223,511	207,725	243,497
Total after-tax	$ (274,802)	$ (257,083)	$ (215,260)	$ (223,610)	$ (242,343)	$ (190,586)	$ (223,973)	$ (237,484)	(970,755)	(894,386)	(1,031,114)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax									63,080	36,443	(8,620)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Available for Sale Assets
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses									84,416	32,165	(37,874)
Net investment income									(4,178)	212	6,164
Total before tax									80,238	32,377	(31,710)
Tax									(16,850)	(6,799)	6,659
Total after-tax									63,388	25,578	(25,051)
Reclassification out of Accumulated Other Comprehensive Income | Pension adjustments:
Unrealized investment (gains) losses on available for sale assets:
Total before tax									(390)	13,754	20,797
Tax									82	(2,889)	(4,366)
Total after-tax									(308)	10,865	16,431
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service cost
Unrealized investment (gains) losses on available for sale assets:
Other operating expense									1,075	1,077	631
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial (gain) loss
Unrealized investment (gains) losses on available for sale assets:
Other operating expense									$ (1,465)	$ 12,677	$ 20,166