Stockholders' Equity - Restricted Stock Vested by or Granted to Non-Executive Directors (Detail) (USD $)	3 Months Ended												39 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Dec. 31, 2013
Equity [Abstract]
Grant Date	October 1, 2013	July 1, 2013	April 1, 2013	January 2, 2013	October 1, 2012	July 2, 2012	April 2, 2012	January 3, 2012	October 3, 2011	July 1, 2011	April 1, 2011	January 3, 2011
Price Per Share	$ 14.16	$ 13.57	$ 13.27	$ 11.68	$ 11.87	$ 12.86	$ 13.93	$ 13.87	$ 13.20	$ 15.26	$ 14.48	$ 17.77
Restricted Stock Granted	6,266	4,606	4,712	5,350	5,263	4,084	3,769	3,785	3,979	3,439	3,627	2,955	51,835
Restricted Stock Vested		(737)	(754)	(856)	(5,263)	(4,084)	(3,769)	(3,785)	(3,979)	(3,439)	(3,627)	(2,955)	(33,248)
Vesting Date					October 1, 2013	July 1, 2013	April 1, 2013	January 2, 2013	October 1, 2012	July 2, 2012	April 2, 2012	January 2, 2012
Restricted Stock Granted and Outstanding	6,266	3,869	3,958	4,494									18,587
Cash Dividends Paid					$ 2,211	$ 2,042	$ 2,356	$ 2,839	$ 3,482	$ 3,439	$ 3,627	$ 2,955	$ 22,951