Summary of Financial Statement Preparation and Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Capitalized interest	$ 1.8	$ 1.3	$ 0.2
Other liabilities for self-insured reserves	$ 18.4	$ 18.7
Minimum [Member] | Ground equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	3 years
Minimum [Member] | facilities, leasehold improvements and office equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	3 years
Maximum [Member] | Ground equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	10 years
Maximum [Member] | facilities, leasehold improvements and office equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	25 years
Boeing 767 and 757 aircraft and flight equipment [Member] | Minimum [Member] | Flight Equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	10 years
Boeing 767 and 757 aircraft and flight equipment [Member] | Maximum [Member] | Flight Equipment [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	18 years