Income Taxes - Summary of Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected federal income tax expense (benefit)		$ 195,569	$ 181,371	$ 290,727
Tax reform adjustment	$ 798	0	(798)	(877,400)
Low income housing investments		(11,605)	(12,240)	(18,515)
Share-based awards		(11,780)	(6,450)	(19,549)
Other		(1,787)	278	(2,878)
Income Tax Expense (Benefit), Total		$ 170,397	$ 162,161	$ (627,615)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected federal income tax expense (benefit)		21.00%	21.00%	35.00%
Tax reform adjustment		0	(0.001)	(1.056)
Low income housing investments		(1.20%)	(1.40%)	(2.20%)
Share-based awards		(1.30%)	(0.70%)	(2.40%)
Other		(0.20%)	0.00%	(0.40%)
Effective Income Tax Rate Reconciliation, Percent, Total		18.30%	18.80%	(75.60%)

Effective Income Tax Rate Reconciliation, Excess Tax Benefit from Share-based Compensation, Percent

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The effective income tax rate reduction resulting from credits associated with low-income housing interests.

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This amount is the reduction in taxes caused by the credits associated with low-income housing interests.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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Amount of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to expense for award under share-based payment arrangement. Excludes expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

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Amount of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations, attributable to Tax Cuts and Jobs Act.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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