Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Valuation allowance as of beginning of year	$ 84,343	$ 67,726	$ 70,236
Increases recorded to income tax provision	20,719	16,617	24,773
Decreases recorded to income tax provision			(27,283)
Valuation allowance as of end of year	$ 105,062	$ 84,343	$ 67,726