Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2015
Income Tax [Line Items]
Tax Cuts and Jobs Act of 2017, Provisional income tax expense (benefit)	$ 10,400
Cumulative effect of a change in accounting principle related to stock-based compensation					$ 2,711
Provision for (benefit from) income taxes		$ (1,836)	$ (1,062)	$ 11,205
Valuation allowance	5,773	9,100
Increase in valuation allowance		3,300	2,100
Unrecognized tax benefits that would impact effective tax rate	3,500		$ 2,800				$ 2,400
State
Income Tax [Line Items]
Operating loss carryforwards		14,000
State | Research and Development Credits [Member]
Income Tax [Line Items]
Tax credit carryforward		10,900
Domestic Tax Authority [Member]
Income Tax [Line Items]
Operating loss carryforwards		48,400
Domestic Tax Authority [Member] | Research and Development Credits [Member]
Income Tax [Line Items]
Tax credit carryforward		$ 12,200
Foreign Tax Authority
Income Tax [Line Items]
Tax credit carryforward	$ 2,200
Accounting Standards Update 2016-09
Income Tax [Line Items]
Cumulative effect of a change in accounting principle related to stock-based compensation						$ 7,745
Retained earnings | Accounting Standards Update 2016-09
Income Tax [Line Items]
Cumulative effect of a change in accounting principle related to stock-based compensation						$ 7,745