Business Segment Information - Major customer markets (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	May 31, 2020	Feb. 29, 2020	Nov. 30, 2019	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	May 31, 2020	May 31, 2019	May 31, 2018
Disaggregation of revenue by major customer markets
Sales	$ 416.5	$ 553.1	$ 560.9	$ 541.5	$ 562.7	$ 529.5	$ 493.3	$ 466.3	$ 2,072.0	$ 2,051.8	$ 1,748.3
Aviation Services
Disaggregation of revenue by major customer markets
Sales									1,964.2	1,920.6	1,635.8
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Sales									1,268.9	1,342.3	1,281.9
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Sales									695.3	578.3	353.9
Expeditionary Services
Disaggregation of revenue by major customer markets
Sales									107.8	131.2	112.5
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Sales									24.3	31.6	37.5
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Sales									$ 83.5	$ 99.6	$ 75.0