Employee Stock Purchase Plan - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of discounted closing price for Employee Stock Purchase Plan	85.00%
Employee Stock Purchase Plan, common stock issued	136,000	114,000	119,000
Employee Stock Purchase Plan, weighted average price per share	$ 39.44	$ 33.76	$ 25.59