Property and Equipment, net Property and Equipment, net - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) a	Dec. 31, 2018 USD ($) airplane	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Depreciation		$ 546.1	$ 547.9	$ 398.2
Finite-lived intangible assets acquired		$ 89.1
Airplanes
Property, Plant and Equipment [Line Items]
Number of airplanes sold | airplane		2
Net book value of airplanes sold		$ 65.3
Proceeds for sale of airplanes		50.6
Loss on disposal		14.7
Land
Property, Plant and Equipment [Line Items]
Payments to acquire land	$ 336.2
Area of land acquired | a	38
Land acquired		$ 247.0
Assets Leased to Others
Property, Plant and Equipment [Line Items]
Area of land acquired | a	16