Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pro forma results
Pro forma tax rate used in calculating taxes on income from continuing operations (as a percent)	28.00%	40.00%	28.00%	40.00%
Revenues	$ 908,902	$ 849,084	$ 2,388,020	$ 2,218,392
Income before provision for income taxes	45,521	3,487	61,709	68,254
Net income (loss) attributable to Primoris	$ 32,691	$ 3,438	$ 40,676	$ 41,774
Weighted average common shares outstanding:
Basic (in shares)	51,403	51,441	51,471	51,491
Diluted (in shares)	51,735	51,707	51,760	51,751
Earnings per share:
Basic (in dollars per share)	$ 0.64	$ 0.07	$ 0.79	$ 0.81
Diluted (in dollars per share)	$ 0.63	$ 0.07	$ 0.79	$ 0.81