Employee Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Cash balance pension plan
Employee benefit plans
Interest crediting rate as a percentage of average 30-year treasury rate	100.00%
Average period of treasury rate used in determination of interest crediting rate (in years)	30
Average interest crediting rate (as a percent)	5.00%
Defined Benefit Pension Plans
Change in benefit obligation:
Benefit obligation at beginning of year	$ 105,091	$ 104,530
Benefit obligations from acquired companies	4,651
Service cost	1,619	2,143	1,528
Interest cost	5,453	5,376	5,578
Plan participants' contributions	400	385
Net actuarial (gain) loss	17,387	(6,415)
Benefits paid	(5,455)	(5,248)
Plan change	351
Curtailment		(1,277)
Translation	(6,537)	5,597
Benefit obligation at end of year	122,960	105,091	104,530
Change in plan assets:
Fair value of plan assets at beginning of year	94,422	78,892
Actual return on plan assets	(8)	9,987
Employer contributions	4,528	4,653
Plan participants' contributions	400	385
Benefits paid	(5,455)	(5,248)
Translation	(6,218)	5,753
Fair value of plan assets at end of year	87,669	94,422	78,892
Funded status at end of year	(35,291)	(10,669)
Amounts recognized in the consolidated balance sheets
Other assets		5,498
Accrued liabilities	(1,887)	(1,610)
Other liabilities and deferred income	(33,404)	(14,557)
Total	(35,291)	(10,669)
Amounts recognized in accumulated other comprehensive loss
Tax on amounts recognized in accumulated other comprehensive loss	20,161	12,661
Actuarial loss	37,281	22,131
Prior service cost	747	564
Total	38,028	22,695
Pension plans with an accumulated benefit obligation in excess of plan assets
Projected benefit obligation	85,158	71,057
Accumulated benefit obligation	85,049	70,903
Fair value of plan assets	50,171	54,890
Accumulated benefit obligation	118,317	100,529
Net Periodic Benefit Cost
Service cost	1,619	2,143	1,528
Interest cost	5,453	5,376	5,578
Expected return on plan assets	(6,559)	(5,871)	(5,936)
Amortization of prior service cost	150	138	132
Recognized net actuarial loss	990	1,219	1,148
Total	$ 1,653	$ 3,005	$ 2,450
Defined Benefit Pension Plans | Outside directors
Employee benefit plans
Benefits paid in cash as percentage of the annual retainer fees payable to active outside director	25.00%
Defined Benefit Pension Plans | Minimum | Outside directors
Employee benefit plans
Minimum age of retirement (in years)	65
Required minimum years of service as a director	5
Defined Benefit Pension Plans | Maximum | Outside directors
Employee benefit plans
Period for which benefits will be payable (in years)	10
Domestic plans
Key assumptions used in the measurement of the company's benefit obligations
Discount rate (as a percent)	4.14%	5.26%
Rate of compensation increase (as a percent)	2.50%	3.50%
Weighted average assumptions used to determine net periodic pension expense
Discount rate (as a percent)	5.26%	5.60%	6.82%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Expected long-term return on plan assets (as a percent)	8.00%	8.00%	8.50%
Actual assets allocation
Equity securities (as a percent)	59.00%	60.00%
Fixed income securities (as a percent)	22.00%	22.00%
Other (fund-of funds hedge fund) (as a percent)	19.00%	18.00%
Total	100.00%	100.00%
Domestic plans | Minimum
Target Assets Allocation
Equity securities (as a percent)	45.00%
Fixed income securities (as a percent)	15.00%
Other (fund-of funds hedge fund) (as a percent)	0.00%
Domestic plans | Maximum
Target Assets Allocation
Equity securities (as a percent)	75.00%
Fixed income securities (as a percent)	45.00%
Other (fund-of funds hedge fund) (as a percent)	25.00%
Non-domestic plans
Key assumptions used in the measurement of the company's benefit obligations
Discount rate (as a percent)	4.11%	5.80%
Rate of compensation increase (as a percent)	3.00%	3.00%
Weighted average assumptions used to determine net periodic pension expense
Discount rate (as a percent)	5.54%	4.50%	5.90%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Expected long-term return on plan assets (as a percent)	5.80%	4.50%	6.50%