Goodwill and Intangibles - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended		72 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2011	Dec. 31, 2017	Dec. 31, 2010
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill acquisitions		$ 33,250,000	$ 18,851,000
Changes in the value of goodwill				$ 0
Goodwill	$ 54,613,000	$ 52,705,000		54,613,000	$ 0
Goodwill impairment	0
2018	16,821,000			16,821,000
2019	16,331,000			16,331,000
2020	16,151,000			16,151,000
2021	15,534,000			15,534,000
2022	$ 13,865,000			$ 13,865,000