Note 6 - Unearned Revenue - Schedule of Unearned Revenue (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020
Unearned Revenue	$ 3,452,021	$ 3,325,737	$ 3,535,639
Water and Mineral Services [Member]
Unearned Revenue	153,051	154,185	130,561
California 1[Member]
Unearned Revenue	963,504	809,450	820,525
Federal [Member]
Unearned Revenue	110,348	132,384	140,494
Heavy Civil [Member]
Unearned Revenue	956,941	974,847	1,456,617
Midwest [Member]
Unearned Revenue	403,077	485,718	326,314
Northwest [Member]
Unearned Revenue	865,100	769,153	661,128
Transportation [Member]
Unearned Revenue	2,075,498	2,064,848	2,626,520
Transportation [Member] | Water and Mineral Services [Member]
Unearned Revenue	0	0	0
Transportation [Member] | California 1[Member]
Unearned Revenue	769,260	627,002	636,385
Transportation [Member] | Federal [Member]
Unearned Revenue	7,303	10,028	16,464
Transportation [Member] | Heavy Civil [Member]
Unearned Revenue	622,491	774,123	1,188,587
Transportation [Member] | Midwest [Member]
Unearned Revenue	107,630	135,655	214,016
Transportation [Member] | Northwest [Member]
Unearned Revenue	568,814	518,040	571,068
Water [Member]
Unearned Revenue	362,713	190,253	227,864
Water [Member] | Water and Mineral Services [Member]
Unearned Revenue	153,051	154,185	130,561
Water [Member] | California 1[Member]
Unearned Revenue	44,066	27,754	61,151
Water [Member] | Federal [Member]
Unearned Revenue	73	100	861
Water [Member] | Heavy Civil [Member]
Unearned Revenue	161,632	6,791	34,961
Water [Member] | Midwest [Member]
Unearned Revenue	0	0	0
Water [Member] | Northwest [Member]
Unearned Revenue	3,891	1,423	330
Specialty [Member]
Unearned Revenue	1,013,810	1,070,636	681,255
Specialty [Member] | Water and Mineral Services [Member]
Unearned Revenue	0	0	0
Specialty [Member] | California 1[Member]
Unearned Revenue	150,178	154,694	122,989
Specialty [Member] | Federal [Member]
Unearned Revenue	102,972	122,256	123,169
Specialty [Member] | Heavy Civil [Member]
Unearned Revenue	172,818	193,933	233,069
Specialty [Member] | Midwest [Member]
Unearned Revenue	295,447	350,063	112,298
Specialty [Member] | Northwest [Member]
Unearned Revenue	$ 292,395	$ 249,690	$ 89,730