Business Segment Information - Geographic Data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2018	May 31, 2017	May 31, 2016
Business Segment Information
Long-lived assets	$ 582.0				$ 615.7				$ 582.0	$ 615.7
Sales	473.5	$ 456.3	$ 420.6	$ 397.9	450.5	$ 407.2	$ 371.3	$ 361.8	1,748.3	1,590.8	$ 1,525.4
Continuing operations
Business Segment Information
Long-lived assets	582.0				517.9				582.0	517.9
Discontinued operations
Business Segment Information
Long-lived assets					97.8					97.8
Aviation Services
Business Segment Information
Sales									1,618.9	1,485.4	1,425.0
Expeditionary Services
Business Segment Information
Sales									129.4	105.4	100.4
United States
Business Segment Information
Long-lived assets	393.1				369.1				393.1	369.1
Europe
Business Segment Information
Long-lived assets	103.3				108.5				103.3	108.5
Other
Business Segment Information
Long-lived assets	$ 85.6				$ 40.3				85.6	40.3
Foreign countries
Business Segment Information
Sales									$ 694.0	$ 595.4	$ 498.0
Foreign countries | Total sales | Geographic concentration
Business Segment Information
Percentage of total sales									39.70%	37.40%	32.60%
U.S. Department of Defense, other U.S. government agencies and their contractors
Business Segment Information
Sales									$ 304.3	$ 321.5	$ 366.1
Percentage of total sales									17.40%	20.20%	24.00%
U.S. Department of Defense, other U.S. government agencies and their contractors | Aviation Services
Business Segment Information
Sales									$ 224.4	$ 260.2	$ 301.1
U.S. Department of Defense, other U.S. government agencies and their contractors | Expeditionary Services
Business Segment Information
Sales									$ 79.9	$ 61.3	$ 65.0