Recent Accounting Developments Recent Accounting Developments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Adjustments for New Accounting Pronouncements [Line Items]
Income tax deficiency	$ 0.5
ASU 2014-09
Adjustments for New Accounting Pronouncements [Line Items]
Expected increase in customer flight miles liability	10.0
Expected reclassification of passenger revenue	$ 1,000.0