Summary of Significant Accounting Policies - Intangibles (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2016	May 31, 2015	May 31, 2014
Amortizable intangible assets:
Gross carrying amount	$ 61.0	$ 57.9
Accumulated amortization	(26.5)	(22.5)
Net carrying amount	34.5	35.4
Unamortized intangible assets:
Trademarks	1.3	1.3
Intangible Assets, Gross (Excluding Goodwill)	62.3	59.2
Intangible Assets, Net (Excluding Goodwill), Total	35.8	36.7
Amortization expense	4.4	4.6	$ 4.7
Estimated aggregate amortization expense for intangible assets in each of the next five fiscal years
2017	4.9
2018	4.7
2019	3.9
2020	3.8
2021	3.5
Customer relationships
Amortizable intangible assets:
Gross carrying amount	22.7	23.4
Accumulated amortization	(10.5)	(9.0)
Net carrying amount	12.2	14.4
Developed technology
Amortizable intangible assets:
Gross carrying amount	11.4	8.0
Accumulated amortization	(4.6)	(3.5)
Net carrying amount	6.8	4.5
Lease agreements
Amortizable intangible assets:
Gross carrying amount	21.5	21.5
Accumulated amortization	(9.9)	(8.7)
Net carrying amount	$ 11.6	12.8
Unamortized intangible assets:
Useful life	18 years
FAA certificates
Amortizable intangible assets:
Gross carrying amount	$ 5.2	5.0
Accumulated amortization	(1.5)	(1.3)
Net carrying amount	$ 3.7	$ 3.7
Unamortized intangible assets:
Useful life	20 years
Trademarks
Amortizable intangible assets:
Gross carrying amount	$ 0.2
Net carrying amount	$ 0.2
Minimum | Customer relationships
Unamortized intangible assets:
Useful life	10 years
Minimum | Developed technology
Unamortized intangible assets:
Useful life	7 years
Maximum | Customer relationships
Unamortized intangible assets:
Useful life	20 years
Maximum | Developed technology
Unamortized intangible assets:
Useful life	15 years