Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 15,716,649	$ 15,460,191	$ 16,250,819
Cumulative effect of change in accounting principle — equity securities			(16,853)
Net gain (loss) during period	143,648	213,401	(39,414)
Ending balance	16,187,283	15,716,649	15,460,191
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	37,380	(147,526)	(44,150)
Cumulative effect of change in accounting principle — equity securities			(16,853)
Net gain (loss) during period	107,222	184,906	(86,523)
Ending balance	144,602	37,380	(147,526)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(342,419)	(261,303)	(304,546)
Net gain (loss) during period	(138,645)	(81,116)	43,243
Ending balance	(481,064)	(342,419)	(261,303)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	98,359	(11,252)	(15,118)
Net gain (loss) during period	175,071	109,611	3,866
Ending balance	273,430	98,359	(11,252)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(206,680)	(420,081)	(363,814)
Cumulative effect of change in accounting principle — equity securities			(16,853)
Net gain (loss) during period	143,648	213,401	(39,414)
Ending balance	$ (63,032)	$ (206,680)	$ (420,081)