Significant Accounting Policies - Property and Equipment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Impairment loss	$ 0
Depreciation and amortization	$ 18,733,000	$ 17,817,000	$ 18,104,000
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of property and equipment	40 years