Income Taxes - Variation Between Expected and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	35.00%	35.00%	35.00%
State income taxes (net of federal benefit)	0.80%	2.00%	3.80%
Benefit of qualified domestic production activities	(2.30%)	(8.90%)	(3.10%)
(Benefit) expense of research and development tax credits	(7.30%)	(48.90%)	0.60%
Increase in valuation allowance	0.00%	0.90%	0.20%
Non deductible book expenses	0.90%	1.80%	0.40%
Changes in deferred tax assets	0.00%	(1.50%)	0.30%
Remeasurement of deferred taxes for changes in state tax law	(2.10%)	0.00%	(5.60%)
Changes in tax reserves	(0.70%)	(0.50%)	(5.30%)
Other	0.00%	(1.10%)	0.60%
Effective income tax rate (benefit)	24.30%	(21.20%)	26.90%