Allowance for Credit Losses - Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Nov. 30, 2021	Aug. 31, 2021	May 31, 2021	Nov. 30, 2020	Aug. 31, 2020	May 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan loss	$ 86,135	$ 89,535	$ 85,532	$ 58,989	$ 57,351	$ 53,125
Financing receivable, allowance for credit loss to outstanding	0.30%		0.30%
Outstanding amount	$ 28,934,814		$ 28,415,107	27,051,163
Credit reserve for unadvanced loan commitments	1,000		1,000
Nonperforming financial instruments
Financing Receivable, Allowance for Credit Loss [Line Items]
Outstanding amount	$ 219,444		$ 237,497
Collective allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss to outstanding	0.16%		0.15%
Allowance for credit loss, period increase (decrease)	$ 4,000
Asset-specific allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss to outstanding	17.50%		17.41%
Allowance for credit loss, period increase (decrease)	$ (3,000)
RTFC
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan loss	$ 1,618	$ 4,820	$ 4,695	$ 3,239	$ 3,792	$ 4,881
Financing receivable, allowance for credit loss to outstanding	0.37%		1.12%
Outstanding amount	$ 431,641		$ 420,383
RTFC | Nonperforming financial instruments
Financing Receivable, Allowance for Credit Loss [Line Items]
Outstanding amount	$ 0		$ 9,185
RTFC | Collective allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss to outstanding	0.28%		0.28%
RTFC | Asset-specific allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss to outstanding	9.28%		25.75%