Segment Reporting - Reporting (Details) $ in Thousands		3 Months Ended									6 Months Ended	12 Months Ended
	Dec. 22, 2015 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jun. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Aug. 31, 2015 USD ($)
Segment Reporting
Number of operating segments | segment												4
Segment reporting
Total revenues		$ 224,173	$ 120,409	$ 140,494	$ 149,727	$ 148,966	$ 134,773	$ 138,466	$ 137,609			$ 634,803	$ 559,814	$ 530,801
Income (loss) before income tax expense		104,645	$ 3,190	(8,949)	$ 22,340	17,801	$ 12,078	$ 16,688	$ 18,420			121,226	64,987	43,055
Identifiable assets		1,709,022				1,350,849						1,709,022	1,350,849	1,539,472
Capital purchases												11,905	13,798	33,549
Depreciation and amortization												44,151	49,384	53,606
Non-cash share based compensation												16,667	15,359	13,539
Pre-tax gain on sale												107,699
Aggregate amount payable				20,300											$ 20,300
Legal and related costs				$ 2,300								4,900
Facility closing expense										$ 1,600
Restructuring charges														(75)
Duplicate rent charges											$ 2,600			2,600
Non-operating charge , which included a reserve for the remaining lease obligation for the previous midtown Manhattan headquarters										3,900
Long-lived Assets		133,971				146,724						133,971	146,724	135,947
Restructuring accruals recorded in 2012 and 2011
Segment reporting
Reduction in previously recorded restructuring accruals										$ 1,600
Energy research business | Sale of subsidiaries
Segment reporting
Cash proceeds from sale	$ 120,500
Pre-tax gain on sale	$ 107,699
U.S. Operations
Segment reporting
Total revenues												285,230	306,540	317,876
Income (loss) before income tax expense												16,000	35,803	40,433
Identifiable assets		1,304,995				929,062						1,304,995	929,062	1,183,986
Capital purchases												7,748	10,645	28,367
Depreciation and amortization												33,971	38,659	43,017
Non-cash share based compensation												10,725	10,107	9,463
Long-lived Assets		97,226				107,746						97,226	107,746	116,812
Canadian Operations
Segment reporting
Total revenues												63,028	77,633	74,156
Income (loss) before income tax expense												10,643	17,259	10,653
Identifiable assets		76,561				105,898						76,561	105,898	88,503
Capital purchases												2,175	1,656	1,188
Depreciation and amortization												2,247	3,153	3,437
Non-cash share based compensation												1,466	695
Long-lived Assets		6,357				7,070						6,357	7,070	5,676
European Operations
Segment reporting
Total revenues												129,729	127,410	91,600
Income (loss) before income tax expense												28,244	34,081	20,000
Identifiable assets		262,095				249,702						262,095	249,702	202,015
Capital purchases												1,334	1,114	3,323
Depreciation and amortization												6,560	6,422	6,012
Non-cash share based compensation												4,505	2,891	2,074
Long-lived Assets		27,226				29,531						27,226	29,531	11,413
Asia Pacific Operations
Segment reporting
Total revenues												48,179	46,926	45,711
Income (loss) before income tax expense												1,620	(1,690)	(2,126)
Identifiable assets		65,371				66,187						65,371	66,187	64,968
Capital purchases												648	383	671
Depreciation and amortization												1,373	1,150	1,140
Non-cash share based compensation												866	979	854
Long-lived Assets		$ 3,162				$ 2,377						3,162	2,377	2,046
Corporate
Segment reporting
Total revenues												108,637	1,305	1,458
Income (loss) before income tax expense												64,719	(20,466)	(25,905)
Non-cash share based compensation												$ (895)	$ 687	$ 1,148