Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired intangible assets
Fair Value	$ 21,125	$ 3,000	$ 3,850
Pro forma results
Pro forma tax rate used in calculating taxes on income from continuing operations (as a percent)	28.20%	44.20%
Revenues	$ 2,406,062	$ 2,092,872
Income before provision for income taxes	107,055	57,280
Net income attributable to Primoris	$ 73,626	$ 31,415
Weighted average common shares outstanding:
Basic (in shares)	51,481	51,762
Diluted (in shares)	51,741	51,989
Earnings per share:
Basic (in dollars per share)	$ 1.43	$ 0.61
Diluted (in dollars per share)	$ 1.42	$ 0.60
Tradename
Acquired intangible assets
Fair Value	$ 2,150
Tradename | Minimum
Acquired intangible assets
Amortization Period	1 year
Tradename | Maximum
Acquired intangible assets
Amortization Period	3 years
Non-compete agreements
Acquired intangible assets
Fair Value	$ 550		1,350
Non-compete agreements | Minimum
Acquired intangible assets
Amortization Period	2 years
Non-compete agreements | Maximum
Acquired intangible assets
Amortization Period	5 years
Customer relationships
Acquired intangible assets
Fair Value	$ 18,150	$ 3,000	$ 2,500
Customer relationships | Minimum
Acquired intangible assets
Amortization Period	5 years
Customer relationships | Maximum
Acquired intangible assets
Amortization Period	10 years
Other
Acquired intangible assets
Amortization Period	3 years
Fair Value	$ 275