Unvested Restricted Stock Grants Outstanding (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested		(22,933)	(8,333)
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance	526,972	590,661	619,739
Granted	244,060	184,758	169,134
Vested	(138,321)	(215,065)	(168,723)
Forfeited	(45,082)	(33,382)	(29,489)
Ending balance	587,629	526,972	590,661
Restricted stock units | Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted	97,145	86,118	90,634
Restricted stock units | Performance Based Shares | Earned
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested		(22,993)	(8,333)
Restricted stock units | Performance Based Shares | Unearned
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested	(46,998)	(42,334)	(12,000)
Restricted stock units | Non Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted	146,915	98,640	78,500
Vested	(91,323)	(149,738)	(148,390)