Revenues by Client Location (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 125,538	$ 110,078	$ 103,800	$ 94,383	$ 95,127	$ 86,423	$ 80,176	$ 72,802	$ 433,799	$ 334,528	$ 221,824
Reimbursable expenses and other revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									6,417	6,176	3,773
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									197,593	163,068	117,027
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									98,346	70,989	32,584
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									47,507	43,799	31,488
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									30,120	15,870	9,751
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									16,391	7,909	7,239
Kazakhstan
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									11,352	8,845	7,480
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									9,256	2,058
Sweden
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,913	5,292	3,034
Ukraine
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									4,733	891	924
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									3,127	4,031	5,399
Other Countries
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 4,044	$ 5,600	$ 3,125