Business Segment Information - Segment reporting (Details) $ in Millions	12 Months Ended
	May 31, 2021 USD ($) segment	May 31, 2020 USD ($)	May 31, 2019 USD ($)
Business Segment Information
Number of business segments | segment	2
Net sales	$ 1,652.3	$ 2,072.0	$ 2,051.8
Gross profit	275.9	269.2	329.8
Total assets	1,539.7	2,079.0
Capital expenditures	11.3	23.6	17.9
Depreciation and amortization, including amortization of stock-based compensation, Net	45.5	51.0	56.3
Continuing operations
Business Segment Information
Capital expenditures	11.3	23.6	17.4
Discontinued operations
Business Segment Information
Capital expenditures			0.5
Aviation Services
Business Segment Information
Net sales	1,553.7	1,964.2	1,920.6
Gross profit	263.2	267.3	313.6
Total assets	1,357.2	1,502.5
Aviation Services | Continuing operations
Business Segment Information
Capital expenditures	8.1	16.9	15.1
Depreciation and amortization, including amortization of stock-based compensation, Gross	33.7	39.2	37.5
Expeditionary Services
Business Segment Information
Net sales	98.6	107.8	131.2
Gross profit	12.7	1.9	16.2
Total assets	73.0	103.6
Expeditionary Services | Continuing operations
Business Segment Information
Capital expenditures	3.1	5.4	1.4
Depreciation and amortization, including amortization of stock-based compensation, Gross	2.3	3.7	4.1
Corporate | Continuing operations
Business Segment Information
Capital expenditures	0.1	1.3	0.9
Depreciation and amortization, including amortization of stock-based compensation, Gross	9.5	8.1	$ 14.7
Corporate | Discontinued operations
Business Segment Information
Total assets	$ 109.5	$ 472.9