Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Total revenues	$ 303,800	$ 260,401	$ 897,202	$ 769,445
London Operation [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 2,000	$ 2,100	$ 7,800	$ 7,200	$ 9,100