Significant Accounting Policies - Income Taxes (Details)	12 Months Ended
	Dec. 31, 2017
Valuation Allowance [Line Items]
Recovery of demand troughs	4 years
Threshold period for cumulative losses	4 years
Minimum
Valuation Allowance [Line Items]
Economic cycle	7 years
Maximum
Valuation Allowance [Line Items]
Economic cycle	10 years