Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 15,587,529	$ 15,709,694	$ 15,460,191	$ 16,250,819
Net gain during period	175,051	(34,698)	307,123	(137,274)
Ending balance	15,691,827	15,577,885	15,691,827	15,577,885
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(147,526)
Net gain during period			154,444
Ending balance	6,918		6,918
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(261,303)
Net gain during period			6,108
Ending balance	(255,195)		(255,195)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(11,252)
Net gain during period			146,571
Ending balance	135,319		135,319
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(288,009)	(483,243)	(420,081)	(363,814)
Net gain during period			307,123
Ending balance	$ (112,958)	$ (517,941)	$ (112,958)	$ (517,941)