Related-Party Transactions (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	3 Months Ended
Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2012 One Earth Energy [Member]	Jul. 31, 2011 One Earth Energy [Member]
Related Party Transaction, Purchases from Related Party	$ 120.9	$ 127.8	$ 64.4	$ 58.4

Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

No definition available.

Name:	us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration