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Investments - Investment in joint ventures (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Investments
Investment in WMES joint ventures at beginning of the period				$ 41,590
Capital contribution				630	$ 2,623
Earnings from joint ventures		$ 558	$ 269	1,127	$ 819
Distribution				(1,303)
Foreign currency translation adjustment		(304)		(304)
Investment in WMES joint ventures at end of the period		41,740		41,740
WMES
Investments
Investment in WMES joint ventures at beginning of the period				26,672
Capital contribution				630
Earnings from joint ventures				1,052
Distribution				(1,303)
Investment in WMES joint ventures at end of the period		27,051		27,051
CASC
Investments
Investment in WMES joint ventures at beginning of the period				14,918
Capital contribution	$ (15,000)
Earnings from joint ventures				75
Foreign currency translation adjustment				(304)
Investment in WMES joint ventures at end of the period		$ 14,689		$ 14,689