Summary of Significant Accounting Policies - Interest Income Table (Details) - USD ($) $ in Thousands		3 Months Ended				9 Months Ended
		Feb. 29, 2016		Feb. 28, 2015		Feb. 29, 2016		Feb. 28, 2015		May. 31, 2015
Components of interest income
Interest income		$ 253,633		$ 238,740		$ 756,074		$ 711,266
Interest on investments		1,323		2,395		5,905		6,516
Fee income		(279)	[1]	32	[2]	(808)	[1]	185	[1]
Deferred conversion fees		60,000				60,000				$ 70,000
Long-term fixed-rate loans
Components of interest income
Interest income	[2]	240,933		224,770		716,736		669,121
Long-term variable-rate loans
Components of interest income
Interest income		5,077		4,836		14,919		15,099
Line of credit loans
Components of interest income
Interest income		6,335		6,707		18,919		20,335
Restructured loans
Components of interest income
Interest income		163		$ 0		293		$ 10
Non-performing loans
Components of interest income
Interest income		$ 81				$ 110

[1] Primarily related to amortization of loan origination costs and late payment fees. For the three and nine months ended February 29, 2016, it excludes loan upfront and arranger fees, which are not based on interest rates and are included in the fee and other income line of the condensed consolidated statements of operations.

[2] Includes loan conversion fees, which are deferred and recognized in interest income using the effective interest method. Also includes a small portion of conversion fees, which are intended to cover the administrative costs related to the conversion and are recognized into income immediately at conversion