Computation for Basic and Diluted Income per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Earnings Per Share, Basic and Diluted, by Common Class
Net income	$ 6,099	$ 7,019	$ 11,476	$ 13,877
Weighted average basic shares outstanding	2,016	2,019	2,015	2,018
Add: Effect of dilutive securities	3	11	8	12
Weighted average diluted shares outstanding	2,019	2,030	2,023	2,030
Earnings per share:
Basic	$ 3.03	$ 3.48	$ 5.70	$ 6.88
Diluted	$ 3.02	$ 3.46	$ 5.67	$ 6.84