Debt and Available Credit Facilities (Details) - USD ($) $ in Thousands				1 Months Ended	12 Months Ended
	Oct. 31, 2015	Aug. 06, 2015	Dec. 22, 2014	Aug. 31, 2015	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Line of Credit Facility [Line Items]
Amount of financing available under credit facilities					$ 1,100,000
Debt financing costs paid					0	$ 3,362	$ 0
Unused lines of credit					$ 700,000
Weighted average interest rate on total debt outstanding during the period					2.19%	1.88%
Weighted average interest rate on total debt at period end					2.74%	2.12%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings under revolving credit facilities					$ 365,000	$ 605,000
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Optional credit limit increase available on request					350,000
Minimum increments in which optional credit limit increase may be requested					$ 50,000
Debt financing costs paid						3,400
Term of credit facility					5 years
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Line of credit facility fee percentage					0.15%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Line of credit facility fee percentage					0.25%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | LIBOR [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Applicable margin					0.98%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | LIBOR [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Applicable margin					1.50%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Applicable margin					0.00%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Applicable margin					0.45%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Federal Funds Effective Rate [Member]
Line of Credit Facility [Line Items]
Margin rate over reference rate used in determining base rate					0.50%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Eurocurrency Rate [Member]
Line of Credit Facility [Line Items]
Margin rate over reference rate used in determining base rate					1.00%
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Previous Revolving Credit Agreement [Member]
Line of Credit Facility [Line Items]
Amount of financing available under credit facilities					$ 940,000
Line of credit facility, due date					Nov. 02, 2016
Syndicate Bank Group led by Bank of America [Member] | Revolving Credit Facility [Member] | Amended and Extended Revolving Credit Agreement [Member]
Line of Credit Facility [Line Items]
Amount of financing available under credit facilities					$ 1,100,000
Line of credit facility, due date					Mar. 01, 2021
TD Bank, N.A. [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Amount of financing available under credit facilities	$ 50,000
Term of credit facility	364 days
Santander Bank, N.A. [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Amount of financing available under credit facilities		$ 100,000	$ 50,000
Line of credit facility, due date		May 23, 2016
Line of credit facility, amount drawn				$ 50,000
Term of credit facility			364 days
Period for reinstatement of revolving credit facility			30 days
Other Credit Facilities [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings under revolving credit facilities					$ 0	$ 0
Amount of financing available under credit facilities					$ 6,800