Recent accounting pronouncements Revenue, Initial Application Period Cumulative Effect Transition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	$ 38,249		$ 38,249		$ 47,521	$ 43,804
Sales Revenue, Services, Net	22,284	$ 20,216	40,260	$ 40,336
Revenue, Net	146,528	109,020	262,766	223,701
Cost of Services	(18,429)	(16,146)	(34,005)	(33,536)
Cost of Revenue	(115,966)	(77,851)	(202,731)	(161,828)
Gross profit	30,562	31,169	60,035	61,873
Operating Income (Loss)	17,361	22,150	38,402	44,065
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	13,097	19,693	30,560	37,054
Income Tax Expense (Benefit)	(3,905)	(8,794)	(8,377)	(15,587)
Net Income (Loss) Attributable to Parent	$ 9,192	$ 10,899	$ 22,183	$ 21,467
Earnings Per Share, Basic and Diluted	$ 0.48	$ 0.57	$ 1.16	$ 1.12
Inventory, Net	$ 77,972		$ 77,972		51,326	54,147
Deferred Tax Liabilities, Net, Noncurrent	201,682		201,682		193,858	194,084
Retained Earnings (Accumulated Deficit)	522,037		522,037		515,122	514,453
Accumulated Other Comprehensive Income (Loss), Net of Tax	(5,531)		(5,531)		(4,713)	$ (4,710)
Accounts Receivable [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Initial Application Period Cumulative Effect Transition					3,717
Inventories [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Initial Application Period Cumulative Effect Transition					(2,821)
Other Noncurrent Liabilities [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Initial Application Period Cumulative Effect Transition					(226)
Retained Earnings [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Initial Application Period Cumulative Effect Transition					669
AOCI Attributable to Parent [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Initial Application Period Cumulative Effect Transition					$ (3)
Previous Accounting Guidance [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	31,690		31,690
Sales Revenue, Services, Net	20,791		37,470
Revenue, Net	145,035		259,976
Cost of Services	17,415		31,579
Cost of Revenue	114,952		200,305
Gross profit	30,083		59,671
Operating Income (Loss)	16,882		38,038
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	12,618		30,196
Income Tax Expense (Benefit)	3,782		8,284
Net Income (Loss) Attributable to Parent	8,836		21,912
Inventory, Net	80,655		80,655
Adjustments for New Accounting Pronouncement [Member]
Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	6,559		6,559
Sales Revenue, Services, Net	1,493		2,790
Revenue, Net	1,493		2,790
Cost of Services	1,014		2,426
Cost of Revenue	1,014		2,426
Gross profit	479		364
Operating Income (Loss)	479		364
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	479		364
Income Tax Expense (Benefit)	123		93
Net Income (Loss) Attributable to Parent	356		271
Inventory, Net	$ (2,683)		$ (2,683)