Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassifications (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ (2,135)	$ (11,943)	$ 28,752	$ (18,426)
Net investment income	(231,432)	(228,905)	(691,991)	(683,003)
Other operating expense	75,397	74,575	226,693	226,412
Total before tax	(231,538)	(247,330)	(646,700)	(704,191)
Tax	42,593	45,512	119,167	130,370
Total after-tax	(188,945)	(201,818)	(527,533)	(573,729)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax	12,635	(7,032)	39,129	(6,282)
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	10,276	(12,430)	32,133	(18,337)
Net investment income	1,562	1,410	4,927	4,030
Total before tax	11,838	(11,020)	37,060	(14,307)
Tax	(2,486)	2,314	(7,783)	3,004
Total after-tax	9,352	(8,706)	29,277	(11,303)
Reclassification out of Accumulated Other Comprehensive Income | Pension Adjustments
Unrealized investment (gains) losses on available for sale assets:
Total before tax	4,156	2,117	12,471	6,355
Tax	(873)	(443)	(2,619)	(1,334)
Total after-tax	3,283	1,674	9,852	5,021
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	$ 3,998	$ 1,959	$ 11,997	$ 5,881