Recent Accounting Developments Recent Accounting Developments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Adjustments for New Accounting Pronouncements [Line Items]
Income tax deficiency	$ 0.6
Accounting Standards Update 2014-09 [Member]
Adjustments for New Accounting Pronouncements [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Expected (Increase) Decrease in Customer Flight Miles Liability	10.0
Expected increase to passenger revenue after adoption	$ 1,000.0