Accounting Policies - Various (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Impairment of Long-lived Assets
Impairment of Oil and Gas	$ 0
Derivative Instruments and Hedging Activities
Accumulated other comprehensive income	$ 0
Minimum
Depreciation and amortization
Estimated useful lives (in years)	1 year
Maximum
Depreciation and amortization
Estimated useful lives (in years)	8 years
Leasehold improvements | Minimum
Depreciation and amortization
Estimated useful lives (in years)	1 year
Leasehold improvements | Maximum
Depreciation and amortization
Estimated useful lives (in years)	8 years
Office furniture, fixtures and computer equipment | Minimum
Depreciation and amortization
Estimated useful lives (in years)	3 years
Office furniture, fixtures and computer equipment | Maximum
Depreciation and amortization
Estimated useful lives (in years)	7 years
Vehicles
Depreciation and amortization
Estimated useful lives (in years)	5 years
Capitalized interest | Minimum
Capitalized Interest
Expected construction period for capitalization of interest costs on major projects	1 year