Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,250,819	$ 16,486,622	$ 16,173,289
Net gain (loss) during period	(39,414)	(5,149)	(43,009)
Reclassification of income tax effects to retained earnings		64,029
Cumulative effect of change in accounting principle — equity securities	(16,853)
Ending balance	15,460,191	16,250,819	16,486,622
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(44,150)	(16,319)	48,087
Net gain (loss) during period	(86,523)	(19,766)	(64,406)
Reclassification of income tax effects to retained earnings		(8,065)
Cumulative effect of change in accounting principle — equity securities	(16,853)
Ending balance	(147,526)	(44,150)	(16,319)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(304,546)	(272,874)	(296,979)
Net gain (loss) during period	43,243	22,288	24,105
Reclassification of income tax effects to retained earnings		(53,960)
Ending balance	(261,303)	(304,546)	(272,874)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(15,118)	(5,443)	(2,735)
Net gain (loss) during period	3,866	(7,671)	(2,708)
Reclassification of income tax effects to retained earnings		(2,004)
Ending balance	(11,252)	(15,118)	(5,443)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(363,814)	(294,636)	(251,627)
Net gain (loss) during period	(39,414)	(5,149)	(43,009)
Reclassification of income tax effects to retained earnings		(64,029)
Cumulative effect of change in accounting principle — equity securities	(16,853)
Ending balance	$ (420,081)	$ (363,814)	$ (294,636)