Property and Equipment - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 6,000,000	$ 6,500,000	$ 5,200,000
Property and equipment, gross	198,000	260,000
Accumulated depreciation and amortization	113,000	137,000
Asset retirement obligation	175,000	210,000
Computer and Office Equipment
Property Plant And Equipment [Line Items]
Fully depreciated computer and office equipment retired	$ 1,500,000	$ 5,500,000