Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 16,486,622	$ 16,173,289
Net gain (loss) during period	$ 21,486	$ 49,168	24,555	150,606
Ending balance	16,283,537	16,471,514	16,283,537	16,471,514
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			28,338
Net gain (loss) during period			(8,400)
Ending balance	19,938		19,938
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(44,657)
Net gain (loss) during period			23,976
Ending balance	(20,681)		(20,681)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(272,874)
Net gain (loss) during period			8,331
Ending balance	(264,543)		(264,543)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(5,443)
Net gain (loss) during period			648
Ending balance	(4,795)		(4,795)
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			24,555
Ending balance	$ (270,081)	$ (101,021)	$ (270,081)	$ (101,021)