Income Taxes - Net Operating Loss and Tax Credit Carryforwards, Expiration (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	$ 1,007,303
Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	23,067
Year of Expiration 2027 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	6,256
Year of Expiration 2027 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	840
Year of Expiration 2028 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	38,954
Year of Expiration 2028 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	2,108
Year of Expiration 2029 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	4,755
Year of Expiration 2029 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1,149
Year of Expiration 2030 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	1,030
Year of Expiration 2030 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1,162
Year of Expiration 2031 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	605
Year of Expiration 2031 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	3,097
Year of Expiration 2032 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	1,533
Year of Expiration 2032 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	3,666
Year of Expiration 2033 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	37,209
Year of Expiration 2033 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	3,178
Year of Expiration 2034 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	4,353
Year of Expiration 2034 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1,861
Year of Expiration 2035 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	195,416
Year of Expiration 2035 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	752
Year of Expiration 2036 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	293,661
Year of Expiration 2036 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1,739
Year Of Expiration 2037 [Member]
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforwards	423,531
Year Of Expiration 2037 [Member] | Federal Research and Development Tax Credit Carryforwards [Member]
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 3,515