Capital Management - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 31, 2016
Equity [Abstract]
Share repurchase program, new limit amount								$ 25,000,000
Shares repurchased under the program			0
Shares repurchased	1,258,250	1,258,250	1,258,250	1,258,250
Shares repurchased, value	$ 22,370,000	$ 22,370,000	$ 22,370,000	$ 22,370,000
Shares repurchased, average price			$ 17.78
Quarterly dividend per share	$ 0.20	$ 0.18	0.20	$ 0.18	$ 0.15	$ 0.12	$ 0.08
Extraordinary dividends declared per share	$ 3.50	$ 3.25	$ 3.50	$ 3.25	$ 3.00	$ 1.50	$ 0