Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,486,622	$ 16,173,289
Net gain (loss) during period	3,069	101,438
Ending balance	16,213,104	16,355,053
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	28,338
Net gain (loss) during period	(5,232)
Ending balance	23,106
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(44,657)
Net gain (loss) during period	3,876
Ending balance	(40,781)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(272,874)
Net gain (loss) during period	3,972
Ending balance	(268,902)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5,443)
Net gain (loss) during period	453
Ending balance	(4,990)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(294,636)	(251,627)
Net gain (loss) during period	3,069
Ending balance	$ (291,567)	$ (150,189)