Employee Benefit Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Employee Benefit Plans [Line Items]
Grant awards	1,000,000
Maximum number of shares of common stock	80,000
Vested of common stock	1
Issuance of common stock	1
Common stock dividends declared	$ 82,000	$ 249,000	$ 228,000
Compensation expense	757,000	643,000	480,000
Employee voluntary contribution	50.00%
Employee compensation	3.00%
Contributions, net of forfeitures	270,000	239,000	220,000
Number of stock option exercised			5,250
Intrinsic value of stock option exercised			76,000
Number of stock option outstanding	5,000
Stock option exercise price	$ 27.68
Remaining contractual life	5 years
Intrinsic value of stock option outstanding	0
Restricted Stock Units (RSUs) [Member]
Employee Benefit Plans [Line Items]
Restricted stock awarded	52,125	47,625	37,600
RSUs vest starting period from date of grant	1 year
Annual rate of vest of total number of RSUs	20.00%
Fair value of grants recognition period	5 years
Compensation expense	746,000	638,000	466,000
Unrecognized compensation cost	1,825,000
Weighted average period	2 years 7 months 6 days
Number of outstanding RSUs	216,350	170,825	123,200	85,600
Number of vested RSUs	93,225
Intrinsic value of outstanding RSUs	3,907,000
Intrinsic value of vested RSUs	$ 1,684,000
Supplemental Employee Retirement Plan [Member]
Employee Benefit Plans [Line Items]
Executive compensation	10.00%
Employees And Directors [Member] | Restricted Stock Units (RSUs) [Member]
Employee Benefit Plans [Line Items]
Restricted stock awarded	52,125	47,625	37,600
Termination of grant	10 years