Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance
Valuation allowance at beginning of year	$ 256,165	$ 190,174	$ 161,660
Mergers and acquisitions, net	0	(1,416)	1,228
Current year operations	26,619	67,549	28,035
Adoption of ASU 2016-09	17,843	0	0
Provisional impact of the Tax Act	(87,473)	0	0
Changes in foreign tax rates and foreign tax reforms	1,327	0	0
Foreign currency translation adjustment	1,101	(142)	(749)
Valuation allowance at end of year	$ 215,582	$ 256,165	$ 190,174

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Change in the valuation allowance for deferred tax assets as a result of current year operations.

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- Definition

Change in the valuation allowance for deferred tax assets as a result of mergers and acquisitions, net.

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Change in the valuation allowance for deferred tax assets as a result of foreign currency translation adjustments.

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Change in the valuation allowance for deferred tax assets resulting from the adoption of a recently issued accounting pronouncement.

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- Definition

Change in the valuation allowance for deferred tax assets resulting from enacted changes in foreign tax rates.

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- Definition

Valuation Allowance, Tax Cuts and Jobs Act, Change in tax Rate, Deferred Tax Asset, Amount

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