Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (63,600)
Other comprehensive income (loss), net of tax	$ 125,664	$ (9,965)	283,038	$ (152,366)
Ending Balance	219,438		219,438
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(16,103)	117,230
Other comprehensive income (loss) before reclassifications			280,951	(154,403)
Reclassification of amounts included in net income			(134)	62
Other comprehensive income (loss), net of tax			280,817	(154,341)
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				17,557
Ending Balance	264,714	(19,554)	264,714	(19,554)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(47,497)	(37,718)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			2,221	1,975
Other comprehensive income (loss), net of tax			2,221	1,975
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				(8,022)
Ending Balance	(45,276)	(43,765)	(45,276)	(43,765)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(63,600)	79,512
Other comprehensive income (loss) before reclassifications			280,951	(154,403)
Reclassification of amounts included in net income			2,087	2,037
Other comprehensive income (loss), net of tax			283,038	(152,366)
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings				9,535
Ending Balance	$ 219,438	$ (63,319)	$ 219,438	$ (63,319)

Accumulated Other Comprehensive Income (Loss) [Roll Forward]

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- Definition

Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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