Investments in Affiliates Equity Method Investment Summarized Income Statement Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Granite’s interest in affiliates’ net income	$ 7,107	$ 7,177	$ 3,210
Other Affiliates [Member] | Joint Venture Unconsolidated [Member]
Revenue	56,372	56,127	47,457
Gross profit	23,007	22,398	19,117
Income before taxes	17,154	19,117	8,446
Net income	17,154	19,117	8,446
Granite’s interest in affiliates’ net income	$ 7,107	$ 7,177	$ 3,210