Fair Value Measurements (Tables)

12 Months Ended

May 31, 2016

Fair Value Measurements [Abstract]

Financial Assets and Liabilities Measured at Fair Value on Recurring Basis

	May 31, 2016
			Quoted		Significant
			prices in		other		Significant
	Carrying		active		observable		unobservable
	value		markets		inputs		inputs
In millions	(Fair value)		(Level 1)		(Level 2)		(Level 3)
Assets:
Available-for-sale securities:
Corporate bonds	$	131.2	$	—	$	131.2	$	—
General obligation municipal bonds		1,621.6		—		1,621.6		—
Pre-refunded municipal bonds		61.6		—		61.6		—
Revenue municipal bonds		931.8		—		931.8		—
U.S. government agency securities		161.1		—		161.1		—
Variable rate demand notes		1,234.6		—		1,234.6		—
Total available-for-sale securities	$	4,141.9	$	—	$	4,141.9	$	—
Other	$	14.9	$	14.9	$	—	$	—
Liabilities:
Other long-term liabilities	$	14.9	$	14.9	$	—	$	—

	May 31, 2015
			Quoted		Significant
			prices in		other		Significant
	Carrying		active		observable		unobservable
	value		markets		inputs		inputs
In millions	(Fair value)		(Level 1)		(Level 2)		(Level 3)
Assets:
Cash equivalents:
Commercial paper	$	15.0	$	—	$	15.0	$	—
General obligation municipal bonds		55.1		—		55.1		—
Pre-refunded municipal bonds		20.5		—		20.5		—
Revenue municipal bonds		12.3		—		12.3		—
Money market securities		0.2		0.2		—		—
Total cash equivalents	$	103.1	$	0.2	$	102.9	$	—
Available-for-sale securities:
General obligation municipal bonds	$	1,703.7	$	—	$	1,703.7	$	—
Pre-refunded municipal bonds		102.7		—		102.7		—
Revenue municipal bonds		963.6		—		963.6		—
Variable rate demand notes		825.6		—		825.6		—
Total available-for-sale securities	$	3,595.6	$	—	$	3,595.6	$	—
Other	$	14.2	$	14.2	$	—	$	—
Liabilities:
Other long-term liabilities	$	14.2	$	14.2	$	—	$	—