Changes in Equity (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2012	Nov. 30, 2011	Nov. 30, 2012	Nov. 30, 2011
Shareholders Equity [Line Items]
Beginning Balance			$ 747,437
Comprehensive income	36,460	4,331	76,045	30,337
Common shares issued			15,628
Stock-based compensation			7,538
Dividends paid to noncontrolling interest, net of contributions			(5,990)
Cash dividends declared			(18,182)
Ending Balance	822,476		822,476
Additional Paid-in Capital
Shareholders Equity [Line Items]
Beginning Balance			192,338
Common shares issued			15,628
Stock-based compensation			7,538
Ending Balance	215,504		215,504
Accumulated Other Comprehensive Income (Loss)
Shareholders Equity [Line Items]
Beginning Balance			(20,387)
Comprehensive income			8,200
Ending Balance	(12,187)		(12,187)
Retained Earnings
Shareholders Equity [Line Items]
Beginning Balance			525,223
Comprehensive income			65,788
Cash dividends declared			(18,182)
Ending Balance	572,829		572,829
Parent
Shareholders Equity [Line Items]
Beginning Balance			697,174
Comprehensive income			73,988
Common shares issued			15,628
Stock-based compensation			7,538
Cash dividends declared			(18,182)
Ending Balance	776,146		776,146
Noncontrolling Interest
Shareholders Equity [Line Items]
Beginning Balance			50,263
Comprehensive income			2,057
Dividends paid to noncontrolling interest, net of contributions			(5,990)
Ending Balance	$ 46,330		$ 46,330