Reconciliation of Effective Tax Rate to Federal Statutory Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	34.90%	35.00%	35.00%
Federal tax law changes	(82.70%)
State taxes, net of federal benefit	2.80%	2.60%	2.40%
Federal and state incentives	(5.20%)	(0.20%)	(0.50%)
State law changes	1.50%	0.30%	(0.90%)
Permanent differences	0.10%	0.70%	0.40%
Net effective rate	(48.60%)	38.40%	36.40%