Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,486,622	$ 16,173,289	$ 12,335,896
Net gain (loss) during period	(5,149)	(43,009)	(70,633)
Reclassification of income tax effects to retained earnings	(64,029)
Ending balance	16,250,819	16,486,622	16,173,289
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	28,338	9,921	4,518
Net gain (loss) during period	(10,755)	18,417	5,403
Reclassification of income tax effects to retained earnings	(3,115)
Ending balance	14,468	28,338	9,921
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(44,657)	38,166	122,683
Net gain (loss) during period	(9,011)	(82,823)	(84,517)
Reclassification of income tax effects to retained earnings	(4,950)
Ending balance	(58,618)	(44,657)	38,166
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(272,874)	(296,979)	(305,589)
Net gain (loss) during period	22,288	24,105	8,610
Reclassification of income tax effects to retained earnings	(53,960)
Ending balance	(304,546)	(272,874)	(296,979)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5,443)	(2,735)	(2,606)
Net gain (loss) during period	(7,671)	(2,708)	(129)
Reclassification of income tax effects to retained earnings	(2,004)
Ending balance	(15,118)	(5,443)	(2,735)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(294,636)	(251,627)	(180,994)
Net gain (loss) during period	(5,149)	(43,009)	(70,633)
Reclassification of income tax effects to retained earnings	(64,029)
Ending balance	$ (363,814)	$ (294,636)	$ (251,627)