Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassifications (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Unrealized investment (gains) losses on available for sale assets:
Realized (gains) losses	$ 4,790	$ (5,154)	$ 30,887	$ (6,483)
Net investment income	(231,568)	(227,425)	(460,559)	(454,098)
Other operating expense	72,714	79,044	151,296	151,837
Total before tax	(212,241)	(228,760)	(415,162)	(456,861)
Tax	39,193	42,151	76,574	84,858
Total after-tax	(173,048)	(186,566)	(338,588)	(371,911)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized investment (gains) losses on available for sale assets:
Total after-tax	765	1,006	26,494	750
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	(5,063)	(2,237)	21,857	(5,907)
Net investment income	1,873	1,394	3,365	2,620
Total before tax	(3,190)	(843)	25,222	(3,287)
Tax	670	177	(5,297)	690
Total after-tax	(2,520)	(666)	19,925	(2,597)
Reclassification out of Accumulated Other Comprehensive Income | Pension Adjustments
Unrealized investment (gains) losses on available for sale assets:
Total before tax	4,158	2,118	8,315	4,238
Tax	(873)	(446)	(1,746)	(891)
Total after-tax	3,285	1,672	6,569	3,347
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	316	316
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial (gain) loss
Unrealized investment (gains) losses on available for sale assets:
Other operating expense	$ 4,000	$ 1,960	$ 7,999	$ 3,922