Income Taxes - Net Change in Total Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of period	$ (68,052)	$ (58,542)	$ (45,011)
Tax valuation increase	(1,324)	(9,779)	(14,375)
Tax valuation decrease	10,106	269	844
Balance at end of period	$ (59,270)	$ (68,052)	$ (58,542)