Goodwill and Intangible Assets (Details) (USD $)	3 Months Ended		6 Months Ended
	Jan. 26, 2013	Jan. 28, 2012	Jan. 26, 2013	Jan. 28, 2012	Dec. 03, 2012	Jul. 28, 2012
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Goodwill, Gross	$ 458,756,000		$ 458,756,000			$ 370,616,000
Goodwill, Impaired, Accumulated Impairment Loss	(195,767,000)		(195,767,000)			(195,767,000)
Goodwill	262,989,000		262,989,000		88,100,000	174,849,000
Goodwill, Acquired During Period			88,140,000
Goodwill, Impairment Loss			0
Goodwill, Period Increase (Decrease)			88,140,000
Carrying amount					91,100,000
Intangible Assets Gross Excluding Goodwill	187,930,000		187,930,000			96,855,000
Net Intangible Assets	134,306,000		134,306,000			49,773,000
Amortization expense for finite-lived intangible assets	5,000,000	1,600,000	6,500,000	3,300,000
UtiliQuest
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Goodwill						35,600,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)						4,700,000
Margin of Excess of Fair Value Over Carrying Value						30.00%
Customer Relationships
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangibles					71,500,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			15 years
Finite-Lived Intangible Asset, Useful Life			12 years 8 months 12 days
Carrying amount	160,645,000		160,645,000			89,145,000
Accumulated amortization	50,019,000		50,019,000			45,852,000
Contract Backlog
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			2 years 8 months 12 days
Carrying amount	14,410,000		14,410,000			0
Accumulated amortization	2,087,000		2,087,000			0
Trade Names
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangibles					5,200,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			5 years
Finite-Lived Intangible Asset, Useful Life			5 years 6 months
Carrying amount	8,025,000		8,025,000			2,860,000
Accumulated amortization	1,455,000		1,455,000			1,182,000
UtiliQuest trade name
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			0 years
Noncompete Agreements
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			2 years 10 months 24 days
Carrying amount	150,000		150,000			150,000
Accumulated amortization	$ 63,000		$ 63,000			$ 48,000
Minimum | Contract Backlog
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			1 year
Maximum | Contract Backlog
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			4 years