Goodwill and Other Intangibles - Amortization Expense Included in General and Administrative Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$ 1,790	$ 1,605	$ 1,513