Business Segment Information (Details 4) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	$ 152,453,000	$ 137,249,000
Consulting fee revenue (as a percent)	100.00%	100.00%
Total Revenue	171,580,000	150,013,000
Total revenue (as a percent)	100.00%	100.00%
U.S./Canada
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	35,257,000	29,291,000
Consulting fee revenue (as a percent)	23.10%	21.30%
Total Revenue	49,340,000	35,630,000
Total revenue (as a percent)	28.80%	23.80%
Latin America
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	7,809,000	10,748,000
Consulting fee revenue (as a percent)	5.10%	7.80%
Total Revenue	7,813,000	10,916,000
Total revenue (as a percent)	4.60%	7.30%
Europe
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	20,112,000	20,439,000
Consulting fee revenue (as a percent)	13.20%	14.90%
Total Revenue	21,414,000	21,836,000
Total revenue (as a percent)	12.50%	14.60%
Middle East
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	75,586,000	63,135,000
Consulting fee revenue (as a percent)	49.60%	46.00%
Total Revenue	78,074,000	66,844,000
Total revenue (as a percent)	45.50%	44.60%
Africa
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	7,063,000	6,264,000
Consulting fee revenue (as a percent)	4.60%	4.60%
Total Revenue	8,199,000	7,156,000
Total revenue (as a percent)	4.80%	4.80%
Asia/Pacific
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	6,626,000	7,372,000
Consulting fee revenue (as a percent)	4.40%	5.40%
Total Revenue	6,740,000	7,631,000
Total revenue (as a percent)	3.80%	4.90%
U.S.
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	34,423,000	28,378,000
Consulting fee revenue (as a percent)	22.60%	20.70%
Total Revenue	48,475,000	34,741,000
Total revenue (as a percent)	28.30%	23.20%
Non - U.S.
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	118,030,000	108,871,000
Consulting fee revenue (as a percent)	77.40%	79.30%
Total Revenue	123,105,000	115,272,000
Total revenue (as a percent)	71.70%	76.80%
Non - U.S. | Consulting fee revenue | Geographic concentration risk
Consulting Fee Revenue by Geographic Region
Number of countries	0	0
Non - U.S. | Total revenue | Geographic concentration risk
Consulting Fee Revenue by Geographic Region
Number of countries	0	0
United Arab Emirates
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	25,288,000	16,490,000
Consulting fee revenue (as a percent)	16.60%	12.00%
Total Revenue	25,527,000	16,701,000
Total revenue (as a percent)	14.90%	11.10%
Oman
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue		15,657,000
Consulting fee revenue (as a percent)		11.40%
Total Revenue		18,009,000
Total revenue (as a percent)		12.00%
Saudi Arabia
Consulting Fee Revenue by Geographic Region
Consulting Fee Revenue	17,060,000
Consulting fee revenue (as a percent)	11.20%
Total Revenue	$ 17,252,000
Total revenue (as a percent)	10.10%