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, 2016	Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2012	Aug. 27, 2016	Aug. 29, 2015	Aug. 27, 2005
Compensation and Retirement Disclosure [Abstract]
Dollar cap liability reduction	10.00%	10.00%	10.00%	10.00%	10.00%	10.00%
Liability reduction due to plan amendment	$ 28,596	$ 3,960	$ 3,580	$ 4,289	$ 4,598	$ 28,596	$ 3,960	$ (40,414)
Amortization period	6 years 10 months 24 days	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=65897934&loc=d3e1928-114920

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