Income Taxes - Summary of Effective Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Expected income taxes	$ 290,727	$ 270,282	$ 268,165
Increase (reduction) in income taxes resulting from, amount:
Tax reform adjustment	(877,400)	0	0
Low income housing investments	(18,515)	(18,202)	(19,031)
Share-based awards	(19,549)	(18,653)	0
Other	(2,878)	(782)	760
Income tax expense (benefit) from continuing operations	$ (627,615)	$ 232,645	$ 249,894
Expected income taxes	35.00%	35.00%	35.00%
Increase (reduction) in income taxes resulting from, percent:
Tax reform adjustment	(105.60%)	0.00%	0.00%
Low income housing investments	(2.20%)	(2.40%)	(2.50%)
Share-based awards	(2.40%)	(2.40%)	(0.00%)
Other	(0.40%)	(0.10%)	0.10%
Income tax expense (benefit) from continuing operations	(75.60%)	30.10%	32.60%

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

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Effective Income Tax Rate Reconciliation, Tax Cuts And Jobs Act of 2017, Amount

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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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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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