Employee Benefit Plans - Asset allocations in the pension plan (Details)	May 31, 2018	May 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	100.00%
Actual Asset Allocations (as a percent)	100.00%	100.00%
International equity
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	8.00%
Actual Asset Allocations (as a percent)	7.90%	8.30%
Large cap equity
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	26.00%
Actual Asset Allocations (as a percent)	26.50%	26.30%
Small cap equity
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	5.00%
Actual Asset Allocations (as a percent)	5.60%	5.30%
Absolute return strategy funds
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	16.00%
Actual Asset Allocations (as a percent)	15.80%	16.20%
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	45.00%
Actual Asset Allocations (as a percent)	44.20%	43.60%
Long/short equity fund
Defined Benefit Plan Disclosure [Line Items]
Target Asset Allocations (as a percent)	0.00%
Actual Asset Allocations (as a percent)	0.00%	0.30%