Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,173,289	$ 12,335,896	$ 11,305,532
Net gain (loss) during period	(43,009)	(70,633)	(116,835)
Ending balance	16,486,622	16,173,289	12,335,896
Investment securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	9,921	4,518	22,632
Net gain (loss) during period	18,417	5,403	(18,114)
Ending balance	28,338	9,921	4,518
Investment securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	38,166	122,683	11,294
Net gain (loss) during period	(82,823)	(84,517)	111,389
Ending balance	(44,657)	38,166	122,683
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(296,979)	(305,589)	(98,182)
Net gain (loss) during period	24,105	8,610	(207,407)
Ending balance	(272,874)	(296,979)	(305,589)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(2,735)	(2,606)	97
Net gain (loss) during period	(2,708)	(129)	(2,703)
Ending balance	(5,443)	(2,735)	(2,606)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(251,627)	(180,994)	(64,159)
Net gain (loss) during period	(43,009)	(70,633)	(116,835)
Ending balance	$ (294,636)	$ (251,627)	$ (180,994)