Summary of Significant Accounting Policies - Difference Between Federal Statutory Rate and Effective Tax Rate (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Line Items]
Tax at statutory rate	21.00%
Tax effect of:
State income taxes, net of federal benefit	0.70%
Valuation allowance on deferred tax assets	1.50%
Nondeductible stock-based compensation	0.60%
Effective tax rate	23.80%
Predecessor
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Line Items]
Tax at statutory rate		21.00%	35.00%
Tax effect of:
State income taxes, net of federal benefit		3.10%
Valuation allowance on deferred tax assets		(24.50%)
Nondeductible stock-based compensation		(1.00%)
Effective tax rate		(1.40%)