Business Segment Information - Summary of financial information concerning reportable segments (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 1,721,005	$ 1,595,227	$ 728,974
Operating profit (loss)	210,030	226,217	(238,942)
Capital expenditures	242,610	117,509	33,938
Depreciation and amortization	163,120	163,537	217,258
Identifiable assets	1,199,580	1,147,224	1,035,452
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,721,005	1,595,227	728,974
Operating Segments | Technical Services
Segment Reporting Information [Line Items]
Revenues	1,647,213	1,538,351	679,654
Operating profit (loss)	216,703	251,476	(203,804)
Capital expenditures	230,262	106,131	28,380
Depreciation and amortization	150,508	145,507	191,181
Identifiable assets	925,305	896,803	733,008
Operating Segments | Support Services
Segment Reporting Information [Line Items]
Revenues	73,792	56,876	49,320
Operating profit (loss)	4,612	(12,228)	(26,021)
Capital expenditures	10,364	9,949	2,928
Depreciation and amortization	12,174	17,570	25,606
Identifiable assets	78,413	75,568	76,876
Corporate
Segment Reporting Information [Line Items]
Revenues	0	0	0
Operating profit (loss)	(14,629)	(17,561)	(17,037)
Capital expenditures	1,984	1,429	2,630
Depreciation and amortization	438	460	471
Identifiable assets	195,862	174,853	225,568
Gain (loss) on disposition of assets, net
Segment Reporting Information [Line Items]
Revenues	0	0	0
Operating profit (loss)	3,344	4,530	7,920
Capital expenditures	0	0	0
Depreciation and amortization	0	0	0
Identifiable assets	$ 0	$ 0	$ 0