Income taxes - Summary of Activity Related to Unrecognized Tax Benefits, Excluding Federal Tax Benefit of State Tax Deductions (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in unrecognized tax benefits
Balance as of beginning of year	$ 12,843	$ 15,043	$ 17,300
Additions based on tax positions related to the current year	0	40	156
Additions for tax positions of prior years	0	2,631	11
Reductions for tax positions of prior years	(959)	0	(636)
Settlements	(288)	(182)	(852)
Reductions due to lapse of statutes of limitations	(3,546)	(4,689)	(936)
Balance as of end of year	$ 8,050	$ 12,843	$ 15,043

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount of unrecognized tax benefits.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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