Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,187,283	$ 15,716,649
Net gain (loss) during period	(75,446)	416,513
Ending balance	16,446,508	15,815,794
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	144,602
Net gain (loss) during period	(22,406)
Ending balance	122,196
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(481,064)
Net gain (loss) during period	13,189
Ending balance	(467,875)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	273,430
Net gain (loss) during period	(66,229)
Ending balance	207,201
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(63,032)	(206,680)
Net gain (loss) during period	(75,446)
Ending balance	$ (138,478)	$ 209,833