Income taxes - Reconciliation of Effective Tax Rate (Detail)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. statutory tax rate	21.00%	21.00%	21.00%
Increase (decrease) in taxes resulting from:
State taxes (net of federal income tax benefit)	3.30%	3.10%	2.90%
Uncertain tax positions, settlements and lapse of statutes of limitations	(0.10%)	(1.60%)	(0.30%)
Other valuation allowances, tax rate changes, & deferred adjustments	(0.10%)	(1.70%)	(1.00%)
Valuation allowance on equity method investment	0.40%	1.70%	0.00%
Enactment of the Tax Cuts and Jobs Act	0.00%	0.00%	(1.10%)
Non-deductible transactions costs	0.00%	0.30%	0.00%
Net excess benefits or expense on share-based payments	(0.10%)	0.40%	0.10%
Other, net	(0.20%)	0.60%	(0.50%)
Effective tax rate	24.20%	23.80%	21.10%

Effective Income Tax Rate Reconciliation, Nondeductible Expense, Transaction Costs, Percent

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Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent

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Effective Income Tax Rate Reconciliation, Valuation Allowance on Equity Method Investment

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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 changes in the valuation allowance for deferred tax assets.

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Percentage 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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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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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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 income tax contingencies. Includes, but not limited to, domestic tax contingency, foreign tax contingency, state and local tax contingency, and other contingencies.

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