Fair Value Disclosures - Financial Instruments on a Recurring Basis (Details) - USD ($) $ in Thousands	May 31, 2017	May 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 22,219	$ 70,405
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	169,266	139,357
Total assets at fair value	191,485	209,762
Long term accrued liabilities:
Total liabilities at fair value		19,628
Fair Value, Measurements, Recurring | Canadian treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	22,219	70,405
Fair Value, Measurements, Recurring | Interest rate lock agreement
Long term accrued liabilities:
Interest rate lock agreement		19,628
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	169,266	139,357
Total assets at fair value	169,266	139,357
Long term accrued liabilities:
Total liabilities at fair value		0
Fair Value, Measurements, Recurring | Level 1 | Canadian treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Interest rate lock agreement
Long term accrued liabilities:
Interest rate lock agreement		0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total assets at fair value	22,219	70,405
Long term accrued liabilities:
Total liabilities at fair value		19,628
Fair Value, Measurements, Recurring | Level 2 | Canadian treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	22,219	70,405
Fair Value, Measurements, Recurring | Level 2 | Interest rate lock agreement
Long term accrued liabilities:
Interest rate lock agreement		19,628
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total assets at fair value	0	0
Long term accrued liabilities:
Total liabilities at fair value		0
Fair Value, Measurements, Recurring | Level 3 | Canadian treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 0	0
Fair Value, Measurements, Recurring | Level 3 | Interest rate lock agreement
Long term accrued liabilities:
Interest rate lock agreement		$ 0