Summary of Significant Accounting Policies - Intangibles (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2015
Amortizable intangible assets:
Gross carrying amount	$ 61.5	$ 61.0
Accumulated Amortization	(29.8)	(26.5)
Net carrying amount	31.7	34.5
Unamortized intangible assets:
Trademarks	1.1	1.3
Intangible Assets, Gross (Excluding Goodwill)	62.6	62.3
Intangible Assets, Net (Excluding Goodwill), Total	32.8	35.8
Amortization expense	4.3	4.4	$ 4.6
Estimated aggregate amortization expense for intangible assets in each of the next five fiscal years
2018	4.5
2019	3.8
2020	3.5
2021	3.5
2022	2.7
Customer relationships
Amortizable intangible assets:
Gross carrying amount	24.4	22.7
Accumulated Amortization	(11.6)	(10.5)
Net carrying amount	12.8	12.2
Developed technology
Amortizable intangible assets:
Gross carrying amount	8.8	11.4
Accumulated Amortization	(4.7)	(4.6)
Net carrying amount	4.1	6.8
Lease agreements
Amortizable intangible assets:
Gross carrying amount	22.5	21.5
Accumulated Amortization	(11.2)	(9.9)
Net carrying amount	11.3	11.6
FAA certificates
Amortizable intangible assets:
Gross carrying amount	5.6	5.2
Accumulated Amortization	(2.3)	(1.5)
Net carrying amount	$ 3.3	3.7
Unamortized intangible assets:
Useful life	20 years
Trademarks
Amortizable intangible assets:
Gross carrying amount	$ 0.2	0.2
Net carrying amount	$ 0.2	$ 0.2
Minimum | Customer relationships
Unamortized intangible assets:
Useful life	10 years
Minimum | Developed technology
Unamortized intangible assets:
Useful life	7 years
Minimum | Lease agreements
Unamortized intangible assets:
Useful life	5 years
Maximum | Customer relationships
Unamortized intangible assets:
Useful life	20 years
Maximum | Developed technology
Unamortized intangible assets:
Useful life	15 years
Maximum | Lease agreements
Unamortized intangible assets:
Useful life	18 years