Derivative Instruments and Hedging Activities Derivatives Offsetting (Details) - USD ($) $ in Thousands	May 31, 2018	May 31, 2017
Derivative [Line Items]
Derivative assets	$ 244,526	$ 49,481
Derivative liabilities	275,932	385,337
Interest rate swaps
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Liability	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	244,526	49,481
Derivative, Collateral, Obligation to Return Securities	196,633	49,481
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	47,893	0
Derivative liabilities	274,873
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	274,873	385,337
Derivative, Collateral, Right to Reclaim Securities	196,633	49,481
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	78,240	335,856
Treasury Lock
Derivative [Line Items]
Derivative liabilities	1,059	$ 0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	1,059
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 1,059