Other Noncurrent Assets - License Fees (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2011 USD ($)	May 31, 2018 USD ($) aircraft	May 31, 2017 USD ($)	May 31, 2016 USD ($)
Intangible assets
License fees		$ 10.6	$ 14.4
Aircraft | Owned Through Joint Ventures
Investments in Joint Ventures
Number of aircraft sold by joint ventures | aircraft		2
Net cash proceeds from sale of aircraft by joint ventures		$ 7.3
Gain from sale of aircraft by joint venture partners		0.4
Payment of evaluation and inspection services		0.4	1.2	$ 0.9
Exclusive distribution rights | Unison Industries
Intangible assets
Term of agreement	10 years
Amount agreed to be paid	$ 20.0
Amount paid	$ 7.0
Amount payable in each calendar year			1.3
Licensing Agreements | Unison Industries
Intangible assets
License fees		$ 5.6	$ 7.2
Amortization period		10 years
Current portion of the deferred payments		$ 1.3
Long-term portion		$ 2.2