Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Provisional income tax expense	$ 24,200,000
Income taxes computed at the Federal statutory rate		35.00%	35.00%	35.00%
Increase (decrease) in current income tax expense		$ 17,873,000.000	$ 62,900,000
Change in valuation allowance		$ 0	0	$ (64,159,000)
Increase (decrease) in valuation allowance				$ (64,200,000)
NOLs available for future years			106,100,000
Minimum | State jurisdiction
Valuation Allowance [Line Items]
Statute of limitations		3 years
Maximum | State jurisdiction
Valuation Allowance [Line Items]
Statute of limitations		4 years
Valuation Allowance, Operating Loss Carryforwards [Member]
Valuation Allowance [Line Items]
Valuation allowance write-offs			$ 50,000,000
Research Tax Credit Carryforward
Valuation Allowance [Line Items]
Tax credit carryforwards	1,400,000	$ 1,400,000
Prior Tax Years | Research Tax Credit Carryforward
Valuation Allowance [Line Items]
Tax credit carryforwards	$ 600,000	$ 600,000