Basis of Presentation and Accounting Policies - Segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Oct. 25, 2014 subsidiary segment	Oct. 26, 2013
Segment Reporting Information [Line Items]
Number of reportable segments	1
Number of subsidiaries	2
Contract revenues	$ 510,389	$ 512,720
Canada
Segment Reporting Information [Line Items]
Contract revenues	$ 4,700	$ 3,300

Aggregate revenue during the period from services rendered in the normal course of business, after deducting allowances and discounts.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(d))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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