Stockholders' Equity - Restricted Stock Vested by or Granted to Non-Executive Directors (Detail) - USD ($)	3 Months Ended												39 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Equity [Abstract]
Price Per Share (usd per share)	$ 16.46	$ 16.84	$ 19.17	$ 17.36	$ 13.01	$ 15.4	$ 18.17	$ 14.28	$ 14.16	$ 13.57	$ 13.27	$ 11.68
Restricted Stock Granted (shares)	6,242	5,494	4,823	5,328	7,110	6,006	5,093	6,475	6,266	4,606	4,712	5,350	67,505
Restricted Stock Vested or Forfeited (shares)	0	0	0	0	(7,110)	(6,006)	(5,093)	(6,475)	(6,266)	(4,606)	(4,712)	(5,350)	(45,618)
Restricted Stock Granted and Outstanding (shares)	6,242	5,494	4,823	5,328	0	0	0	0	0	0	0	0	21,887
Cash Dividends Paid					$ 3,555	$ 3,003	$ 2,547	$ 3,238	$ 3,133	$ 1,935	$ 1,979	$ 2,247	$ 21,637