Income Taxes - Summary of Rollforward of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation Allowance, Deferred Tax Asset [Roll Forward]
Valuation allowance, beginning balance	$ 761,728	$ 507,274	$ 491,356
Net changes in deferred tax assets and liabilities	989,390	254,454	15,918
Valuation allowance, ending balance	$ 1,751,118	$ 761,728	$ 507,274