Summary Of Restricted Stock And Restricted Stock Units Granted (Detail) (USD $) In Thousands, except Share data, 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]
Restricted stock, Shares	149,720	173,553	121,923
Restricted stock units, Shares forfeited			2,013
Restricted stock units, Forfeited shares price per share			$ 29.84
Executives Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, Shares	60,000	167,250	112,500
Restricted stock, Price per share	46.12	44.39	$ 30.87
Restricted stock, Aggregate value	2,767	7,424	$ 3,473
Percent vested	0.00%	20.00%	40.00%
Directors Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, Shares	9,720	6,303	9,423
Restricted stock, Price per share	43.74	40.45	$ 30.85
Restricted stock, Aggregate value	425	255	291
Percent vested	100.00%	100.00%	100.00%
Directors Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, Shares	9,719	13,063	15,443	[1]
Restricted stock, Price per share	43.74	40.49	$ 29.95
Restricted stock, Aggregate value	425	529	$ 463
Percent vested	100.00%	100.00%	100.00%

[1] 2,013 shares at $29.84 per share were later forfeited in 2010.