Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 15,460,191	$ 16,250,819
Net gain during period	132,072	(102,576)
Ending balance	15,587,529	15,709,694
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(147,526)
Net gain during period	84,591
Ending balance	(62,935)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(261,303)
Net gain during period	2,285
Ending balance	(259,018)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(11,252)
Net gain during period	45,196
Ending balance	33,944
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(420,081)	(363,814)
Net gain during period	132,072
Ending balance	$ (288,009)	$ (483,243)