Note 3 - Restatement - Effects of Restatement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Cash and cash equivalents	$ 242,604	$ 200,263	$ 262,273
Short-term marketable securities	5,000	36,049	27,799
Receivables, net ($28,320, $29,564 and $24,990 related to CCJVs)	477,718	380,985	547,417
Contract assets ($17,584, $25,034 and $9,354 related to CCJVs)	226,518	213,023	211,441
Inventories	98,765	96,862	88,885
Equity in construction joint ventures	190,458	219,908	193,110
Other current assets ($16,078, $13,350 and $11,795 related to CCJVs)	60,001	62,755	46,016
Total current assets	1,301,064	1,209,845	1,376,941
Property and equipment, net ($30,047, $31,136 and $35,377 related to CCJVs)	534,958	552,504	542,297
Long-term marketable securities	0	30,000	5,000
Investments in affiliates	73,249	81,034	84,176
Goodwill	248,339	259,695	264,279
Right of use assets	72,945	71,480	72,534
Deferred income taxes, net	51,675	30,488	50,158
Other noncurrent assets	102,145	123,557	106,703
Total assets	2,384,375	2,358,603	2,502,088
Current maturities of long-term debt	8,253	47,281	8,244
Accounts payable ($58,475, $57,795 and $37,853 related to CCJVs)	312,105	216,150	400,775
Contract liabilities ($47,509, $20,994 and $46,804 related to CCJVs)	133,811	104,260	95,737
Accrued expenses and other current liabilities ($2,458, $2,415 and $3,577 related to CCJVs)	355,393	308,086	337,300
Total current liabilities	809,562	675,777	842,056
Long-term debt	355,911	333,290	356,108
Long-term lease liabilities	57,985	60,237	58,618
Deferred income taxes, net	3,318	4,913	3,754
Other long-term liabilities	57,795	59,867	63,136
Commitments and contingencies (Note 18)
Preferred stock, $0.01 par value, authorized 3,000,000 shares, none outstanding		0
Common stock, $0.01 par value, authorized 150,000,000 shares; issued and outstanding: 45,592,292 shares as of March 31, 2020, 45,503,805 shares as of December 31, 2019 and 46,812,366 shares as of March 31, 2019	457	468	456
Additional paid-in capital	551,189	566,497	549,307
Accumulated other comprehensive loss	(6,538)	(1,081)	(2,645)
Retained earnings	522,639	610,302	594,353
Total Granite Construction Incorporated shareholders’ equity	1,067,747	1,176,186	1,141,471
Non-controlling interests	32,057	48,333	36,945
Total equity	1,099,804	1,224,519	1,178,416	$ 1,289,354
Total liabilities and equity	2,384,375	2,358,603	2,502,088
Total revenue	635,927	581,813
Total cost of revenue	612,128	580,675
Gross profit (loss)	23,799	1,138
Selling, general and administrative expenses	78,381	80,155
Acquisition and integration expenses	0	1,848
Gain on sales of property and equipment	(623)	(1,900)
Operating loss	(78,372)	(78,965)
Interest income	(1,291)	(2,816)
Interest expense	4,994	4,014
Equity in income of affiliates, net	(46)	(1,290)
Other expense (income), net	5,219	(1,762)
Total other income	8,876	(1,854)
Loss before benefit from income taxes	(87,248)	(77,111)
Benefit from income taxes	(14,710)	(17,350)
Net loss	(72,538)	(59,761)	$ (59,761)
Amount attributable to non-controlling interests	7,168	(2,709)
Net loss attributable to Granite Construction Incorporated	$ (65,370)	$ (62,470)
Basic (in dollars per share)	$ (1.44)	$ (1.34)
Diluted (in dollars per share)	$ (1.44)	$ (1.34)
Basic (in shares)	45,520	46,699
Diluted (in shares)	45,520	46,699
Net loss	$ (72,538)	$ (59,761)
Depreciation, depletion and amortization	28,447	29,055
Gain on sales of property and equipment, net	(623)	(1,900)
Stock-based compensation	2,398	5,748
Equity in net loss from unconsolidated joint ventures	11,816	20,384
Net income from affiliates	(46)	(1,290)
Receivables	71,040	103,484
Contract assets, net	22,997	(33,345)
Inventories	(9,880)	(8,238)
Contributions to unconsolidated construction joint ventures	(13,767)	(26,933)
Distributions from unconsolidated construction joint ventures and affiliates	2,939	330
Other assets, net	(12,138)	(11,648)
Accounts payable	(87,979)	(40,588)
Accrued expenses and other current liabilities, net	10,333	(11,662)
Net cash used in operating activities	(20,125)	(36,364)
Transportation [Member]
Total revenue	350,901	301,964
Total cost of revenue	325,532	318,312
Water [Member]
Total revenue	101,657	99,082
Total cost of revenue	92,310	91,136
Specialty [Member]
Total revenue	133,039	139,124
Total cost of revenue	143,758	125,826
Materials [Member]
Total revenue	50,330	41,643
Total cost of revenue	$ 50,528	45,401
Previously Reported [Member]
Cash and cash equivalents		200,263
Short-term marketable securities		36,049
Receivables, net ($28,320, $29,564 and $24,990 related to CCJVs)		368,215
Contract assets ($17,584, $25,034 and $9,354 related to CCJVs)		260,250
Inventories		96,862
Equity in construction joint ventures		300,489
Other current assets ($16,078, $13,350 and $11,795 related to CCJVs)		54,590
Total current assets		1,316,718
Property and equipment, net ($30,047, $31,136 and $35,377 related to CCJVs)		552,504
Long-term marketable securities		30,000
Investments in affiliates		81,034
Goodwill		259,695
Right of use assets		71,480
Deferred income taxes, net		0
Other noncurrent assets		128,349
Total assets		2,439,780
Current maturities of long-term debt		47,281
Accounts payable ($58,475, $57,795 and $37,853 related to CCJVs)		216,966
Contract liabilities ($47,509, $20,994 and $46,804 related to CCJVs)		90,752
Accrued expenses and other current liabilities ($2,458, $2,415 and $3,577 related to CCJVs)		265,102
Total current liabilities		620,101
Long-term debt		333,290
Long-term lease liabilities		60,237
Deferred income taxes, net		0
Other long-term liabilities		64,219
Commitments and contingencies (Note 18)
Preferred stock, $0.01 par value, authorized 3,000,000 shares, none outstanding		0
Common stock, $0.01 par value, authorized 150,000,000 shares; issued and outstanding: 45,592,292 shares as of March 31, 2020, 45,503,805 shares as of December 31, 2019 and 46,812,366 shares as of March 31, 2019		468
Additional paid-in capital		566,497
Accumulated other comprehensive loss		(626)
Retained earnings		746,100
Total Granite Construction Incorporated shareholders’ equity		1,312,439
Non-controlling interests		49,494
Total equity		1,361,933
Total liabilities and equity		2,439,780
Total revenue		619,801
Total cost of revenue		579,323
Gross profit (loss)		40,478
Selling, general and administrative expenses		81,155
Acquisition and integration expenses		3,323
Gain on sales of property and equipment		(1,900)
Operating loss		(42,100)
Interest income		(2,816)
Interest expense		4,014
Equity in income of affiliates, net		(1,290)
Other expense (income), net		(1,762)
Total other income		(1,854)
Loss before benefit from income taxes		(40,246)
Benefit from income taxes		(9,165)
Net loss		(31,081)
Amount attributable to non-controlling interests		(3,493)
Net loss attributable to Granite Construction Incorporated		$ (34,574)
Basic (in dollars per share)		$ (0.74)
Diluted (in dollars per share)		$ (0.74)
Basic (in shares)		46,699
Diluted (in shares)		46,699
Net loss		$ (31,081)
Depreciation, depletion and amortization		28,846
Gain on sales of property and equipment, net		(1,900)
Stock-based compensation		5,748
Equity in net loss from unconsolidated joint ventures		(455)
Net income from affiliates		0
Receivables		105,086
Contract assets, net		(55,550)
Inventories		(8,238)
Contributions to unconsolidated construction joint ventures		(26,933)
Distributions from unconsolidated construction joint ventures and affiliates		330
Other assets, net		(4,189)
Accounts payable		(34,110)
Accrued expenses and other current liabilities, net		(13,918)
Net cash used in operating activities		(36,364)
Previously Reported [Member] | Transportation [Member]
Total revenue		338,210
Total cost of revenue		316,960
Previously Reported [Member] | Water [Member]
Total revenue		99,255
Total cost of revenue		91,136
Previously Reported [Member] | Specialty [Member]
Total revenue		140,693
Total cost of revenue		125,826
Previously Reported [Member] | Materials [Member]
Total revenue		41,643
Total cost of revenue		45,401
Revision of Prior Period, Adjustment [Member]
Cash and cash equivalents		0
Short-term marketable securities		0
Receivables, net ($28,320, $29,564 and $24,990 related to CCJVs)		10,566
Contract assets ($17,584, $25,034 and $9,354 related to CCJVs)		(45,011)
Inventories		0
Equity in construction joint ventures		(74,557)
Other current assets ($16,078, $13,350 and $11,795 related to CCJVs)		7,606
Total current assets		(101,396)
Property and equipment, net ($30,047, $31,136 and $35,377 related to CCJVs)		0
Long-term marketable securities		0
Investments in affiliates		0
Goodwill		0
Right of use assets		0
Deferred income taxes, net		26,608
Other noncurrent assets		0
Total assets		(74,788)
Current maturities of long-term debt		0
Accounts payable ($58,475, $57,795 and $37,853 related to CCJVs)		0
Contract liabilities ($47,509, $20,994 and $46,804 related to CCJVs)		14,561
Accrued expenses and other current liabilities ($2,458, $2,415 and $3,577 related to CCJVs)		43,035
Total current liabilities		57,596
Long-term debt		0
Long-term lease liabilities		0
Deferred income taxes, net		0
Other long-term liabilities		0
Commitments and contingencies (Note 18)
Preferred stock, $0.01 par value, authorized 3,000,000 shares, none outstanding		0
Common stock, $0.01 par value, authorized 150,000,000 shares; issued and outstanding: 45,592,292 shares as of March 31, 2020, 45,503,805 shares as of December 31, 2019 and 46,812,366 shares as of March 31, 2019		0
Additional paid-in capital		0
Accumulated other comprehensive loss		0
Retained earnings		(132,325)
Total Granite Construction Incorporated shareholders’ equity		(132,325)
Non-controlling interests		(59)
Total equity		(132,384)
Total liabilities and equity		(74,788)
Total revenue		(28,192)
Total cost of revenue		6,208
Gross profit (loss)		(34,400)
Selling, general and administrative expenses		0
Acquisition and integration expenses		0
Gain on sales of property and equipment		0
Operating loss		(34,400)
Interest income		0
Interest expense		0
Equity in income of affiliates, net		0
Other expense (income), net		0
Total other income		0
Loss before benefit from income taxes		(34,400)
Benefit from income taxes		(7,627)
Net loss		(26,773)
Amount attributable to non-controlling interests		59
Net loss attributable to Granite Construction Incorporated		$ (26,714)
Basic (in dollars per share)		$ (0.57)
Diluted (in dollars per share)		$ (0.57)
Basic (in shares)		46,699
Diluted (in shares)		46,699
Net loss		$ (26,773)
Depreciation, depletion and amortization		0
Gain on sales of property and equipment, net		0
Stock-based compensation		0
Equity in net loss from unconsolidated joint ventures		17,879
Net income from affiliates		0
Receivables		0
Contract assets, net		16,479
Inventories		0
Contributions to unconsolidated construction joint ventures		0
Distributions from unconsolidated construction joint ventures and affiliates		0
Other assets, net		(7,585)
Accounts payable		0
Accrued expenses and other current liabilities, net		0
Net cash used in operating activities		0
Revision of Prior Period, Adjustment [Member] | Transportation [Member]
Total revenue		(28,022)
Revision of Prior Period, Adjustment [Member] | Transportation [Member] | Accounting Standards Update 2016-02 [Member]
Total cost of revenue		6,208
Revision of Prior Period, Adjustment [Member] | Water [Member]
Total revenue		(170)
Total cost of revenue		0
Revision of Prior Period, Adjustment [Member] | Specialty [Member]
Total revenue		0
Total cost of revenue		0
Revision of Prior Period, Adjustment [Member] | Materials [Member]
Total revenue		0
Total cost of revenue		0
Other Adjustments [Member]
Cash and cash equivalents		0
Short-term marketable securities		0
Receivables, net ($28,320, $29,564 and $24,990 related to CCJVs)		2,204
Contract assets ($17,584, $25,034 and $9,354 related to CCJVs)		(2,216)
Inventories		0
Equity in construction joint ventures		(6,024)
Other current assets ($16,078, $13,350 and $11,795 related to CCJVs)		559
Total current assets		(5,477)
Property and equipment, net ($30,047, $31,136 and $35,377 related to CCJVs)		0
Long-term marketable securities		0
Investments in affiliates		0
Goodwill		0
Right of use assets		0
Deferred income taxes, net		3,880
Other noncurrent assets		(4,792)
Total assets		(6,389)
Current maturities of long-term debt		0
Accounts payable ($58,475, $57,795 and $37,853 related to CCJVs)		(816)
Contract liabilities ($47,509, $20,994 and $46,804 related to CCJVs)		(1,053)
Accrued expenses and other current liabilities ($2,458, $2,415 and $3,577 related to CCJVs)		(51)
Total current liabilities		(1,920)
Long-term debt		0
Long-term lease liabilities		0
Deferred income taxes, net		4,913
Other long-term liabilities		(4,352)
Commitments and contingencies (Note 18)
Preferred stock, $0.01 par value, authorized 3,000,000 shares, none outstanding		0
Common stock, $0.01 par value, authorized 150,000,000 shares; issued and outstanding: 45,592,292 shares as of March 31, 2020, 45,503,805 shares as of December 31, 2019 and 46,812,366 shares as of March 31, 2019		0
Additional paid-in capital		0
Accumulated other comprehensive loss		(455)
Retained earnings		(3,473)
Total Granite Construction Incorporated shareholders’ equity		(3,928)
Non-controlling interests		(1,102)
Total equity		(5,030)
Total liabilities and equity		(6,389)
Total revenue		(9,796)
Total cost of revenue		(4,856)
Gross profit (loss)		(4,940)
Selling, general and administrative expenses		(1,000)
Acquisition and integration expenses		(1,475)
Gain on sales of property and equipment		0
Operating loss		(2,465)
Interest income		0
Interest expense		0
Equity in income of affiliates, net		0
Other expense (income), net		0
Total other income		0
Loss before benefit from income taxes		(2,465)
Benefit from income taxes		(558)
Net loss		(1,907)
Amount attributable to non-controlling interests		725
Net loss attributable to Granite Construction Incorporated		$ (1,182)
Basic (in dollars per share)		$ (0.03)
Diluted (in dollars per share)		$ (0.03)
Basic (in shares)		46,699
Diluted (in shares)		46,699
Net loss		$ (1,907)
Depreciation, depletion and amortization		209
Gain on sales of property and equipment, net		0
Stock-based compensation		0
Equity in net loss from unconsolidated joint ventures		2,960
Net income from affiliates		(1,290)
Receivables		(1,602)
Contract assets, net		5,726
Inventories		0
Contributions to unconsolidated construction joint ventures		0
Distributions from unconsolidated construction joint ventures and affiliates		0
Other assets, net		126
Accounts payable		(6,478)
Accrued expenses and other current liabilities, net		2,256
Net cash used in operating activities		0
Other Adjustments [Member] | Transportation [Member]
Total revenue		(8,224)
Total cost of revenue		(4,856)
Other Adjustments [Member] | Water [Member]
Total revenue		(3)
Total cost of revenue		0
Other Adjustments [Member] | Specialty [Member]
Total revenue		(1,569)
Total cost of revenue		0
Other Adjustments [Member] | Materials [Member]
Total revenue		0
Total cost of revenue		$ 0