Income taxes - Additional information (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) agreement shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Tax Credit Carryforward [Line Items]
Tax cuts and jobs act of 2017, change in tax rate, Estimated Additional Income Tax Expense	$ 334,619		$ 316,813
Tax cuts and jobs act of 2017, change in tax rate, remeasurement of deferred tax assets and liabilities	334,022
Tax cuts and jobs act of 2017, change in tax rate, mandatory repatriation	597
Current income tax rate (in percentage)		21.70%	87.10%
Liability related to uncertain tax positions	2,608	$ 300	$ 2,608
Income tax examination, settlement		2,625
Unrecognized tax benefit and related indemnification asset		2,967
Released provision for income taxes		$ 342
Number of tax receivable agreements | agreement		2
Applicable tax savings (in percentage)		85.00%
Percentage of remaining tax savings (in percentage)		15.00%
Income tax rate maximum tax liability (in percentage)		3.50%
Other income (expense) reflecting change in tax benefit obligation		$ (4,765)	317,350	$ (72)
Deferred tax asset	412,091	419,297	412,091
Deferred tax liability	5,938	6,759	5,938
Tax benefit obligation	431,360	429,233	431,360
Continuing LLC Owners
Tax Credit Carryforward [Line Items]
Decrease in deferred tax assets		721	24,371
Deferred tax asset	394,108	27,565	394,108
Deferred tax liability	$ 341,089	$ 23,526	$ 341,089
Class A Common Stock | Continuing LLC Owners
Tax Credit Carryforward [Line Items]
Number of shares exchanged (in shares) | shares		1,736,020	25,842,004
Other expense
Tax Credit Carryforward [Line Items]
Expense for indemnification asset		$ 342