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, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in unrecognized tax benefits
Balance at beginning of year	$ 15,043	$ 17,300	$ 19,491
Additions based on tax positions related to the current year	40	156	213
Additions for tax positions of prior years	2,631	11	162
Reductions for tax positions of prior years	0	(636)	(1,214)
Settlements	(182)	(852)	0
Reductions due to lapse of statutes of limitations	(4,689)	(936)	(1,352)
Balance at end of year	$ 12,843	$ 15,043	$ 17,300