Equity-Based Compensation - Shares of Time-Vested Restricted Stock Granted and Vesting Schedules of Restricted Stock Awards (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	241,551
At Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	6,955
After Three Years Of Service [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	234,596
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	169,081	152,636	225,985
Restricted Stock Awards [Member] | At Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	6,955
Restricted Stock Awards [Member] | After Three Years Of Service [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	162,126
Performance Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	72,470	62,874	149,936
Performance Share Awards [Member] | At Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	0
Performance Share Awards [Member] | After Three Years Of Service [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted	72,470