Derivatives - Rating Triggers (Details) - USD ($) $ in Thousands		Nov. 30, 2019	May 31, 2019
Derivative [Line Items]
Derivative, Notional Amount		$ 10,435,741	$ 10,843,280
Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		7,610,281
Assets Needed for Immediate Settlement, Aggregate Fair Value		(381,645)
Assets Received for Immediate Settlement Aggregate Fair Value		0
Net Asset Needed for Immediate Settlement Aggregate Fair Value		(381,645)
Moody's, A3 Rating Standard Poor's A- rating | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[1]	47,955
Assets Needed for Immediate Settlement, Aggregate Fair Value	[1]	(9,457)
Assets Received for Immediate Settlement Aggregate Fair Value	[1]	0
Net Asset Needed for Immediate Settlement Aggregate Fair Value	[1]	(9,457)
Moodys Baa 1 Rating Standard Poor's BBB Plus Rating | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		6,861,619
Assets Needed for Immediate Settlement, Aggregate Fair Value		(346,099)
Assets Received for Immediate Settlement Aggregate Fair Value		0
Net Asset Needed for Immediate Settlement Aggregate Fair Value		(346,099)
Moody's Baa 2 Rating Standard Poor's BBB Rating | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount	[2]	479,629
Assets Needed for Immediate Settlement, Aggregate Fair Value	[2]	(13,704)
Assets Received for Immediate Settlement Aggregate Fair Value	[2]	0
Net Asset Needed for Immediate Settlement Aggregate Fair Value	[2]	(13,704)
Moodys Baa3 Rating Standard Poor's BB Rating | Interest rate swaps
Derivative [Line Items]
Derivative, Notional Amount		221,078
Assets Needed for Immediate Settlement, Aggregate Fair Value		(12,385)
Assets Received for Immediate Settlement Aggregate Fair Value		0
Net Asset Needed for Immediate Settlement Aggregate Fair Value		$ (12,385)

[1] Rating trigger for CFC falls below A3/A-, while rating trigger for counterparty falls below Baa1/BBB+ by Moody’s or S&P, respectively.

[2] Rating trigger for CFC falls to or below Baa2/BBB, while rating trigger for counterparty falls to or below Ba2/BB+ by Moody’s or S&P, respectively.