Joint Venture - Operating Results (Details) (Joint venture, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Joint venture
Schedule of Equity Method Investments [Line Items]
Revenues	$ 259,487	$ 230,351	$ 257,776
Net income	15,555	13,720	14,603
Current assets	93,679	82,827	94,241
Non-current assets	26,377	25,615	26,099
Current liabilities	11,896	10,548	14,315
Non-current liabilities	35,469	16,103	27,845
Members’ equity	72,691	81,791	76,360
Capital expenditures	3,042	1,836	5,220
Depreciation and amortization	$ 2,294	$ 2,217	$ 2,034