Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal, State and Local Tax [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	$ 35,889	$ 24,537	$ 10,712
Increases as a result of tax positions taken during the period	13,019	12,237	8,108
Decreases as a result of settlements with taxing authorities	(332)		(1,515)
Unrealized tax benefits acquired in a business combination			7,232
Decreases as a result of tax positions in prior years	(3,144)	(885)
Ending Balance	45,432	35,889	24,537
Accrued Interest [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	7,915	7,969	3,869
Increases as a result of tax positions taken in prior years	1,379	656	807
Decreases as a result of settlements with taxing authorities	(168)		(274)
Unrealized tax benefits acquired in a business combination			3,567
Decreases as a result of tax positions in prior years	(3,475)	(710)
Ending Balance	5,651	7,915	7,969
Unrecognized Income Tax Benefits [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	43,804	32,506	14,581
Increases as a result of tax positions taken during the period	13,019	12,237	8,108
Increases as a result of tax positions taken in prior years	1,379	656	807
Decreases as a result of settlements with taxing authorities	(500)		(1,789)
Unrealized tax benefits acquired in a business combination			10,799
Decreases as a result of tax positions in prior years	(6,619)	(1,595)
Ending Balance	51,083	$ 43,804	$ 32,506
Less: Federal, state and local income tax benefits	(10,727)
Net unrecognized tax benefits that, if recognized, would impact the effective income tax rate	$ 40,356