Schedule of Employee Stock Purchase Plan Period (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	2,485,604
Prior To Two Thousand Ten
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	1,851,032
Price	14.23
January 1, 2011 to March 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	38,752
Price	29.75
April 1, 2011 to June 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	54,323
Price	22.64
July 1, 2011 to September 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	94,301
Price	12.60
October 1, 2011 to December 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	103,815
Price	9.27
January 1, 2012 T to March 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	82,619
Price	12.29
April 1, 2012 to June 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	90,411
Price	10.58
July 1, 2012 to September 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	83,269
Price	11.75
October 1, 2012 to December 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issued	87,082
Price	9.61