Other Credit Arrangements - Additional Information (Detail)	12 Months Ended
	Jan. 31, 2015 USD ($)	Apr. 30, 2016 USD ($) Entity
Line Of Credit Facility [Line Items]
Borrowing agreement, number of financial institution | Entity		1
Credit Facilities | Credit Agreement
Line Of Credit Facility [Line Items]
Line of credit facility, expiration period	5 years
Borrowing agreements with financial institutions	$ 400,000,000
Stand-by Letters of Credit | Credit Agreement
Line Of Credit Facility [Line Items]
Letters of credit outstanding amount		$ 8,000,000
Credit Agreement Loans | Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding borrowings		0
Demand letter of credit facilities | Borrowing Agreements
Line Of Credit Facility [Line Items]
Borrowing agreements with financial institutions		5,000,000
Outstanding borrowings		$ 0