Accounting Policies - Useful Lives (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	1 year
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	8 years
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	1 year
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	8 years
Office furniture, fixtures and computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	3 years
Office furniture, fixtures and computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	7 years
Vehicles
Property, Plant and Equipment [Line Items]
Estimated useful lives (in years)	5 years