Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ 15,460,191	$ 16,250,819	$ 16,486,622
Cumulative effect of change inaccounting principle — equity securities	[1]		(16,853)
Net gain (loss) during period		213,401	(39,414)	(5,149)
Reclassification of income tax effects to retained earnings				64,029
Ending balance		15,716,649	15,460,191	16,250,819
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(147,526)	(44,150)	(16,319)
Cumulative effect of change inaccounting principle — equity securities			(16,853)
Net gain (loss) during period		184,906	(86,523)	(19,766)
Reclassification of income tax effects to retained earnings				(8,065)
Ending balance		37,380	(147,526)	(44,150)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(261,303)	(304,546)	(272,874)
Net gain (loss) during period		(81,116)	43,243	22,288
Reclassification of income tax effects to retained earnings				(53,960)
Ending balance		(342,419)	(261,303)	(304,546)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(11,252)	(15,118)	(5,443)
Net gain (loss) during period		109,611	3,866	(7,671)
Reclassification of income tax effects to retained earnings				(2,004)
Ending balance		98,359	(11,252)	(15,118)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(420,081)	(363,814)	(294,636)
Cumulative effect of change inaccounting principle — equity securities			(16,853)
Net gain (loss) during period		213,401	(39,414)	(5,149)
Reclassification of income tax effects to retained earnings				(64,029)
Ending balance		$ (206,680)	$ (420,081)	$ (363,814)

[1] Beginning January 1, 2018, equity securities with readily determinable market values are required to be measured at fair value with changes in fair value recognized in the income statement. Prior to that date, such changes in fair value were reflected in other comprehensive income. Separate presentation of investment securities with an other-than-temporary impairment change is no longer required.