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Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jan. 01, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tax benefit due to performance share unit vesting			$ 1,900
Pretax losses	$ 29,131	$ 21,027	40,290	$ 27,076
Change in tax rate effect on deferred tax liability					$ 16,200
Transition tax					3,700
Increase in global tax rate	0.60%
Accumulated other comprehensive loss	$ (6,340)		(6,340)		(4,366)
Retained earnings	306,375		306,375		$ 274,562	$ 274,836
Accounting Standards Update 2018-02
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accumulated other comprehensive loss	(350)		(350)
Retained earnings	350		$ 350
Europe | Solar Racking Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pretax losses	$ 2,000

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Amount of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to excess tax benefit for share-based compensation cost.

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