CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income	$ 75,801	$ 66,200	$ 68,248
Adjustments to reconcile net income to net cash provided by operating activities:
Non-cash restructuring gains	(939)	(1,000)	(1,044)
Depreciation, depletion and amortization	66,345	64,375	64,309
Gain on sales of property and equipment	(4,182)	(8,358)	(8,286)
Change in deferred income taxes	(4,824)	9,842	28,258
Stock-based compensation	15,764	13,383	8,763
Equity in net loss (income) from unconsolidated joint ventures	14,634	(15,614)	(43,374)
Net Income from affiliates	(7,107)	(7,177)	(3,210)
Changes in assets and liabilities:
Receivables	(60,272)	(75,756)	(32,877)
Costs and estimated earnings in excess of billings, net	(26,066)	2,100	(22,374)
Inventories	(7,252)	308	13,367
Contributions to unconsolidated construction joint ventures	(16,937)	(11,795)	(69,313)
Distributions from unconsolidated construction joint ventures	39,955	19,344	53,367
Prepaid and other assets, net	13,211	(13,873)	(1,078)
Accounts payable	36,716	37,731	8,363
Accrued expenses and other current liabilities	11,348	(6,564)	3,859
Net cash provided by operating activities	146,195	73,146	66,978
Investing activities
Purchases of marketable securities	(124,543)	(129,685)	(104,971)
Maturities of marketable securities	120,000	50,000	29,260
Proceeds from called marketable securities	0	55,000	75,000
Purchases of property and equipment ($18,309 million, $17,810 million and $0 related to CCJVs)	(67,695)	(90,970)	(44,179)
Proceeds from sales of property and equipment	10,202	12,946	13,148
Collection of notes receivable	1,052	4,331	943
Other investing activities, net	1,798	1,988	92
Net cash used in investing activities	(59,186)	(96,390)	(30,707)
Financing activities
Proceeds from long-term debt	25,000	30,000	30,000
Debt principal payments	(45,000)	(45,025)	(46,763)
Cash dividends paid	(20,687)	(20,563)	(20,445)
Purchases of common stock	(6,977)	(5,227)	(3,777)
Contributions from non-controlling partners	11,500	5,250	7,462
Distributions to non-controlling partners	(7,109)	(5,258)	(6,992)
Other financing activities	649	557	1,119
Net cash used in financing activities	(42,624)	(40,266)	(39,396)
Cash and cash equivalents at beginning of year	189,326	252,836	255,961
Cash and cash equivalents at end of year	233,711	189,326	252,836
Increase (decrease) in cash and cash equivalents	44,385	(63,510)	(3,125)
Cash paid during the period for:
Interest	11,446	13,392	14,601
Income taxes	33,948	29,872	4,298
Other non-cash activities:
Performance guarantees	5,497	17,596	(10,306)
Non-cash investing and financing activities:
Restricted stock units issued, net of forfeitures (See Note 13)	11,505	21,101	6,220
Accrued cash dividends	5,183	5,151	5,124
Accrued equipment purchases	(1,945)	(3,865)	2,891
Joint Venture Consolidated [Member]
Changes in assets and liabilities:
Net cash provided by operating activities	36,900	37,800	(16,400)
Joint Venture Consolidated [Member] | Consolidated Construction Joint Venture [Member]
Investing activities
Purchases of property and equipment ($18,309 million, $17,810 million and $0 related to CCJVs)	18,309,000	17,810,000	$ 0
Financing activities
Cash and cash equivalents at beginning of year	73,115
Cash and cash equivalents at end of year	$ 94,359	$ 73,115