Share-Based Compensation Plans - Share-Based Compensation Amounts Included in Net Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense	$ 1,419	$ 1,450	$ 1,155
Less: tax benefit	569	581	463
Share-Based Compensation amounts included in net income	850	869	692
Directors Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense	443	407	283
Performance Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense	$ 976	$ 1,043	$ 872