Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassifications (Detail) - 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
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses									$ 3,737	$ (20,621)	$ (9,274)
Net investment income	$ (235,071)	$ (231,432)	$ (231,568)	$ (228,991)	$ (227,456)	$ (228,905)	$ (227,425)	$ (226,673)	(927,062)	(910,459)	(882,512)
Other operating expense									301,038	304,825	279,585
Total before tax									(896,684)	(931,279)	(863,671)
Tax									(164,911)	(170,397)	(162,161)
Total after-tax	$ (204,240)	$ (188,945)	$ (173,048)	$ (165,540)	$ (187,061)	$ (201,818)	$ (186,566)	$ (185,345)	(731,773)	(760,790)	(701,466)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax									39,058	(4,236)	10,536
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									26,345	(19,352)	(5,715)
Net investment income									6,464	5,515	3,957
Total before tax									32,809	(13,837)	(1,758)
Tax									(6,890)	2,906	369
Total after-tax									25,919	(10,931)	(1,389)
Reclassification out of Accumulated Other Comprehensive Income | Pension Adjustments
Unrealized investment (gains) losses on available for sale assets:
Total before tax									16,632	8,474	15,095
Tax									(3,493)	(1,779)	(3,170)
Total after-tax									13,139	6,695	11,925
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service cost
Unrealized investment (gains) losses on available for sale assets:
Other operating expense									632	631	535
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial (gain) loss
Unrealized investment (gains) losses on available for sale assets:
Other operating expense									$ 16,000	$ 7,843	$ 14,560