Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassifications (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ 29,155	$ (19,789)	$ 66,845	$ (56,600)
Net investment income	(245,625)	(238,975)	(733,101)	(713,103)
Total before tax	(229,983)	(230,795)	(648,594)	(694,832)
Tax	43,204	41,924	120,450	127,826
Total after-tax	(186,779)	(188,871)	(528,144)	(567,006)
Other operating expense	88,140	80,385	262,150	240,750
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax	12,309	(7,425)	32,143	(12,243)
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	12,256	(16,269)	29,741	(35,925)
Net investment income	(114)	1,670	630	4,829
Total before tax	12,142	(14,599)	30,371	(31,096)
Tax	(2,550)	3,066	(6,378)	6,530
Total after-tax	9,592	(11,533)	23,993	(24,566)
Reclassification out of Accumulated Other Comprehensive Income | Pension Adjustments
Unrealized investment (gains) losses on available for sale assets:
Total before tax	3,438	5,198	10,316	15,598
Tax	(721)	(1,090)	(2,166)	(3,275)
Total after-tax	2,717	4,108	8,150	12,323
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service cost
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	158	158	474	474
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial (gain) loss
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	$ 3,280	$ 5,040	$ 9,842	$ 15,124