Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 15,691,827	$ 15,577,885	$ 15,460,191	$ 16,250,819
Net gain during period	32,650	(24,458)	339,773	(161,732)
Ending balance	15,779,811	15,436,085	15,779,811	15,436,085
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(147,526)
Net gain during period			177,049
Ending balance	29,523		29,523
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(261,303)
Net gain during period			9,162
Ending balance	(252,141)		(252,141)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(11,252)
Net gain during period			153,562
Ending balance	142,310		142,310
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(112,958)	(517,941)	(420,081)	(363,814)
Net gain during period			339,773
Ending balance	$ (80,308)	$ (542,399)	$ (80,308)	$ (542,399)