Compensation Programs - Schedule of Performance-Based Stock Awards (Detail) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share	$ 13.02	$ 13.02
Total Original Grants		654,000
2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share		$ 12.87
Total Original Grants		225,000
2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share		$ 11.39
Total Original Grants		225,000
2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share		$ 11.64
Total Original Grants		60,000
2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share		$ 13.23
Total Original Grants		72,000
2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant Date Fair Value Per Share		$ 13.59
Total Original Grants		72,000
Performance-Based Stock Awards [Member] | 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of Grant		2008
Grant Date Fair Value Per Share		$ 10.18
Total Original Grants		140,658
Expiration date of grant		Dec. 31, 2015
Performance-Based Stock Awards [Member] | 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of Grant		2009
Grant Date Fair Value Per Share		$ 14.33
Total Original Grants		110,917
Expiration date of grant		Dec. 31, 2016
Performance-Based Stock Awards [Member] | 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of Grant		2010
Grant Date Fair Value Per Share		$ 12.44
Total Original Grants		128,766
Expiration date of grant		Dec. 31, 2017
Performance-Based Stock Awards [Member] | 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of Grant		2011
Grant Date Fair Value Per Share		$ 12.72
Total Original Grants		132,490
Expiration date of grant		Dec. 31, 2018
Performance-Based Stock Awards [Member] | 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of Grant		2012
Grant Date Fair Value Per Share		$ 11.67
Total Original Grants		145,399
Expiration date of grant		Dec. 31, 2019
Remaining Shares With Initial Measurement Periods 2013 [Member] | Performance-Based Stock Awards [Member] | 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2013 [Member] | Performance-Based Stock Awards [Member] | 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		52,915
Remaining Shares With Initial Measurement Periods 2013 [Member] | Performance-Based Stock Awards [Member] | 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		61,508
Remaining Shares With Initial Measurement Periods 2013 [Member] | Performance-Based Stock Awards [Member] | 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2013 [Member] | Performance-Based Stock Awards [Member] | 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2014 [Member] | Performance-Based Stock Awards [Member] | 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2014 [Member] | Performance-Based Stock Awards [Member] | 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2014 [Member] | Performance-Based Stock Awards [Member] | 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		61,499
Remaining Shares With Initial Measurement Periods 2014 [Member] | Performance-Based Stock Awards [Member] | 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		63,722
Remaining Shares With Initial Measurement Periods 2014 [Member] | Performance-Based Stock Awards [Member] | 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2015 [Member] | Performance-Based Stock Awards [Member] | 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2015 [Member] | Performance-Based Stock Awards [Member] | 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2015 [Member] | Performance-Based Stock Awards [Member] | 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2015 [Member] | Performance-Based Stock Awards [Member] | 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		63,718
Remaining Shares With Initial Measurement Periods 2015 [Member] | Performance-Based Stock Awards [Member] | 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		69,853
Remaining Shares With Initial Measurement Periods 2016 [Member] | Performance-Based Stock Awards [Member] | 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2016 [Member] | Performance-Based Stock Awards [Member] | 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2016 [Member] | Performance-Based Stock Awards [Member] | 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2016 [Member] | Performance-Based Stock Awards [Member] | 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares
Remaining Shares With Initial Measurement Periods 2016 [Member] | Performance-Based Stock Awards [Member] | 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards vested, shares		69,849