Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,720,304	$ 15,945,105	$ 16,187,283	$ 15,716,649
Net gain (loss) during period	(33,445)	(69,127)	(147,387)	382,183
Ending balance	17,528,771	16,101,013	17,528,771	16,101,013
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			144,602
Net gain (loss) during period			(39,808)
Ending balance	104,794		104,794
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(481,064)
Net gain (loss) during period			45,482
Ending balance	(435,582)		(435,582)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			273,430
Net gain (loss) during period			(153,061)
Ending balance	120,369		120,369
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(176,974)	244,630	(63,032)	(206,680)
Net gain (loss) during period			(147,387)
Ending balance	$ (210,419)	$ 175,503	$ (210,419)	$ 175,503