Income Taxes - Reconciliations of Statutory U.S. Federal Income Tax Rates to Our Effective Tax Rate for Continuing Operations (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory rate	35.00%	35.00%	35.00%
Tax reform	3.10%	0.00%	0.00%
State tax, net of federal benefit	2.60%	3.10%	3.00%
Reverse federal impact of indemnification adjustments	2.50%	(0.70%)	0.10%
Unrecognized tax benefits, U.S. federal and state, net of federal benefit	(2.00%)	1.60%	(0.50%)
Excess tax benefits/deficiencies for employee stock-based compensation, federal and state, net of federal benefit	(1.70%)	0.00%	0.00%
Impact of state rate change on net deferred tax liabilities, net of federal benefit	0.60%	(0.50%)	0.50%
State, valuation allowance adjustments on net operating losses	0.20%	1.00%	(0.20%)
Other, net	0.90%	0.10%	(0.40%)
Effective tax rate	41.20%	39.60%	37.50%