Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassifications (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ (68,633)	$ 3,737	$ (20,621)
Net investment income	952,447	927,062	910,459
Total before tax	(912,390)	(896,684)	(931,279)
Tax	167,431	164,911	170,397
Total after-tax	(744,959)	(731,773)	(760,790)
Other operating expense	322,029	301,038	304,825
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax	(8,620)	39,058	(4,236)
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	(37,874)	26,345	(19,352)
Net investment income	6,164	6,464	5,515
Total before tax	(31,710)	32,809	(13,837)
Tax	(6,659)	6,890	(2,906)
Total after-tax	(25,051)	25,919	(10,931)
Reclassification out of Accumulated Other Comprehensive Income | Pension Adjustments
Unrealized investment (gains) losses on available for sale assets:
Total before tax	20,797	16,632	8,474
Tax	4,366	3,493	1,779
Total after-tax	16,431	13,139	6,695
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service cost
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	631	632	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	$ 20,166	$ 16,000	$ 7,843