Employee and Retiree Benefits Postretirement health care liability reduction due to plan amendments (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2012	Aug. 29, 2015	Aug. 30, 2014	Aug. 27, 2005
Compensation and Retirement Disclosure [Abstract]
Dollar cap liability reduction	10.00%	10.00%	10.00%	10.00%	10.00%
Liability reduction due to plan amendment	$ 3,960	$ 3,580	$ 4,289	$ 4,598	$ 3,960	$ 3,580	$ 40,414
Amortization period	7 years 1 month 6 days	7 years 4 months	7 years 6 months	7 years 9 months 20 days			11 years 6 months

The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Plan Amendment

-URI http://asc.fasb.org/extlink&oid=49173939

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)(7)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanPlanAmendments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration