Summary of Significant Accounting Policies - Intangibles (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2016
Amortizable intangible assets:
Gross carrying amount	$ 60.0	$ 57.8
Accumulated Amortization	(33.3)	(28.3)
Net carrying amount	26.7	29.5
Unamortized intangible assets:
Trademarks	1.1	1.1
Intangible Assets, Gross (Excluding Goodwill)	61.1	58.9
Intangible Assets, Net (Excluding Goodwill), Total	27.8	30.6
Amortization expense	4.7	4.2	$ 4.4
Estimated aggregate amortization expense for intangible assets in each of the next five fiscal years
2019	4.1
2020	3.7
2021	3.7
2022	2.9
2023	2.3
Customer relationships
Amortizable intangible assets:
Gross carrying amount	26.3	24.4
Accumulated Amortization	(13.9)	(11.6)
Net carrying amount	12.4	12.8
Developed technology
Amortizable intangible assets:
Gross carrying amount	9.1	8.8
Accumulated Amortization	(5.9)	(4.7)
Net carrying amount	3.2	4.1
Lease agreements
Amortizable intangible assets:
Gross carrying amount	22.5	22.5
Accumulated Amortization	(12.6)	(11.2)
Net carrying amount	9.9	11.3
FAA certificate
Amortizable intangible assets:
Gross carrying amount	1.9	1.9
Accumulated Amortization	(0.9)	(0.8)
Net carrying amount	$ 1.0	1.1
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