Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,446,508	$ 15,815,794	$ 16,187,283	$ 15,716,649
Net gain (loss) during period	(38,496)	34,797	(113,942)	451,310
Ending balance	16,720,304	15,945,105	16,720,304	15,945,105
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			144,602
Net gain (loss) during period			(30,494)
Ending balance	114,108		114,108
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(481,064)
Net gain (loss) during period			29,996
Ending balance	(451,068)		(451,068)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			273,430
Net gain (loss) during period			(113,444)
Ending balance	159,986		159,986
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(138,478)	209,833	(63,032)	(206,680)
Net gain (loss) during period			(113,942)
Ending balance	$ (176,974)	$ 244,630	$ (176,974)	$ 244,630