Income Taxes - Unrecognized Tax Benefit Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 8,566	$ 8,152	$ 7,838
Additions based on tax positions related to current year	880	459	368
Reductions to tax provision of prior years	(24)	(45)	(54)
Balance at end of year	9,422	$ 8,566	$ 8,152
Federal
Net Operating Losses and Tax Credit Carryforwards
Net operating losses, amount	224,536
Tax credits, amount	6,433
State
Net Operating Losses and Tax Credit Carryforwards
Net operating losses, amount	110,802
Tax credits, amount	7,970
Foreign
Net Operating Losses and Tax Credit Carryforwards
Net operating losses, amount	$ 382