Borrowings (Narrative) (Details) (USD $)	3 Months Ended	9 Months Ended				9 Months Ended						9 Months Ended				0 Months Ended	9 Months Ended
	Jan. 31, 2013	Jan. 31, 2013	Aug. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Jan. 31, 2013 Minimum [Member]	Jan. 31, 2013 Maximum [Member]	Jan. 31, 2013 LIBOR [Member] Minimum [Member]	Jan. 31, 2013 LIBOR [Member] Maximum [Member]	Jan. 31, 2013 PRIME [Member] Minimum [Member]	Jan. 31, 2013 PRIME [Member] Maximum [Member]	Jan. 31, 2013 Fiscal Quarter Ending April 30, July 31 and October 31 [Member]	Jan. 31, 2013 Fiscal Quarter Ending On January 31 [Member]	Jan. 31, 2013 Senior Notes, 5.500%, Due November 2022 [Member]	Oct. 25, 2012 Senior Notes, 5.500%, Due November 2022 [Member]	Nov. 26, 2012 Senior Notes, 7.875% Due January 2013 [Member]	Jan. 31, 2013 Senior Notes, 7.875% Due January 2013 [Member]
Line of Credit Facility [Line Items]
Commercial paper borrowings	$ 424,967,000	$ 424,967,000			$ 230,947,000
Long-term debt	906,012,000	906,012,000		409,115,000	409,241,000										500,000,000
Long-term note, interest rate														5.50%			7.875%
Senior notes, maturity date														Nov. 01, 2022			Jan. 01, 2013
Call and repayment price																623,000,000
Make-whole premium																5,800,000
Senior notes, accrued interest																17,200,000
Gain (loss) on extinguishment of debt	(5,800,000)
Line of credit agreement period		5 years
Committed line of credit amount			$ 1,500,000,000
Annual interest rate of facility								0.75%	1.45%	0.00%	0.45%
Line of Credit Facility, Commitment Fee Percentage						0.125%	0.30%
Threshold of debt to EBITDA ratio												3.50%	3.75%
Interest coverage ratio		2.50%