Business Combinations - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Percentage of voting equity interests acquired	100.00%
Goodwill acquired expected to be tax deductible	$ 6,100,000
Amortization of intangibles	7,128,000	$ 6,416,000
Income tax expense (benefit)	2,243,000	3,783,000
Administrative [Member]
Business Acquisition [Line Items]
Acquisition-related costs	500,000	600,000
Alpha [Member]
Business Acquisition [Line Items]
Increase in goodwill	2,100,000
Combined Business Acquisitions [Member]
Business Acquisition [Line Items]
Amortization of intangibles	67,000	700,000
Income tax expense (benefit)	$ 23,000	$ 600,000