Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 63,060	$ 56,854
Initial deferred tax liability related to 2018 convertible debt	12,032	0
Unrecognized tax benefits	2,983	$ 2,141	$ 1,101	$ 343
Uncertain tax positions that would impact the effective tax rate	100
Domestic Tax Authority | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	221,500
Operating loss carryforwards, Sec. 382 limitation	4,200
Net operating loss carryforwards	217,300
Operating loss carryforwards, subject to expiration	170,600
Operating loss carryforwards, not subject to expiration	52,100
State and Local Jurisdiction | Arizona Department of Revenue
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	197,400
Convertible debt
Operating Loss Carryforwards [Line Items]
Initial deferred tax liability related to 2018 convertible debt	$ 14,000