Note 4 - Revisions in Estimates - Summary of Impact of Revisions in Estimates to Gross Profit (Details) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares
Gross Profit, Total		$ 119,935	$ 125,997	$ 300,199	$ 238,139
Decrease to project profitability		(41,327)	87,085	(24,915)	177,088
Decrease to net income/increase to net loss		(32,423)	98,357	(22,847)	171,868
Decrease to net income/increase to net loss attributable to Granite Construction Incorporated		$ (35,043)	$ 91,162	$ (23,309)	$ 153,127
Decrease to net income/increase to net loss per diluted share attributable to common shareholders (1) (in dollars per share) | $ / shares		$ (0.73)	$ 2.00	$ (0.49)	$ 3.36
Estimated Cost Recovery of Customer Affirmative Claims and Back Charges [Member]
Number of projects with downward estimate changes		2	3	5	6
Decrease to project profitability		$ 16,600	$ 32,200	$ 48,200	$ 107,500
Decrease to net income/increase to net loss		13,000	21,700	37,700	72,600
Amounts attributable to non-controlling interests		5,500	8,900	10,000	26,300
Decrease to net income/increase to net loss attributable to Granite Construction Incorporated		$ 7,500	$ 12,800	$ 27,700	$ 46,300
Decrease to net income/increase to net loss per diluted share attributable to common shareholders (1) (in dollars per share) | $ / shares	[1]	$ 0.16	$ 0.28	$ 0.58	$ 1.01
Estimated Cost Recovery of Customer Affirmative Claims and Back Charges [Member] | Minimum [Member]
Gross Profit, Total		$ 5,700	$ 7,200	$ 5,500	$ 6,500
Estimated Cost Recovery of Customer Affirmative Claims and Back Charges [Member] | Maximum [Member]
Gross Profit, Total		$ 10,900	$ 17,800	$ 16,200	$ 37,600

[1] The prior period amounts have been adjusted to correct an immaterial disclosure error in the previously issued September 30, 2020 condensed consolidated financial statements.