Investments - Narrative (Details) - USD ($)	12 Months Ended
	May 31, 2017		May 31, 2016		May 31, 2015
Investments [Abstract]
Investments	$ 164,788,000	[1]	$ 124,952,000	[2]
Cash surrender value of life insurance policies	144,000,000		108,100,000
Equity method investments	15,800,000		14,500,000
Cost method investments	5,000,000		2,400,000
Dividends received on equity method investment	0		0		$ 5,247,000
Cash received from sale of equity method investment					35,200,000
Gain on sale of stock of an equity method investment, net of tax					13,600,000
Losses due to impairment	$ 0		$ 0		$ 0

[1] Investments include inter company investment activity. Corp 2 and Subsidiary Guarantors hold $29.0 million and $135.8 million, respectively, of the $164.8 million consolidated net investments.

[2] Investments include inter company investment activity. Corp 2 and Subsidiary Guarantors hold $15.5 million and $109.5 million, respectively, of the $125.0 million consolidated net investments.