Reserve for Unfunded Commitments - Summary of Activity in Reserve for Unfunded Commitments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 2,719	$ 2,475	$ 2,145
Change in reserve for unfunded commitments	445	244	330
Balance at end of period	$ 3,164	$ 2,719	$ 2,475