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Revenues:Effect of change related to ASU 2014-09 (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Balance Sheet [Abstract]
Retained earnings	$ 405,871,000		$ 405,871,000				$ 361,243,000
Receivables, trade	312,659,000		312,659,000		$ 247,952,000		246,452,000
Accrued liabilities, other	77,953,000		77,953,000		$ 57,286,000		$ 55,786,000
Income Statement [Abstract]
Sales	1,338,512,000		3,929,485,000
Materials, labor, and other operating expenses (excluding depreciation)	1,163,020,000	$ 1,045,742,000	3,366,716,000	$ 2,872,711,000
Selling and distribution expenses	93,395,000	$ 87,520,000	273,592,000	$ 243,509,000
Accounting Standards Update 2014-09 [Member] \| Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Balance Sheet [Abstract]
Receivables, trade	310,959,000		310,959,000
Accrued liabilities, other	76,253,000		76,253,000
Income Statement [Abstract]
Sales	1,331,788,000		3,908,778,000
Materials, labor, and other operating expenses (excluding depreciation)	1,156,903,000		3,347,412,000
Selling and distribution expenses	92,788,000		272,189,000
Accounting Standards Update 2014-09 [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Balance Sheet [Abstract]
Retained earnings						$ 0
Receivables, trade	1,700,000		1,700,000			1,500,000
Accrued liabilities, other	1,700,000		1,700,000			$ 1,500,000
Income Statement [Abstract]
Sales	6,724,000		20,707,000
Materials, labor, and other operating expenses (excluding depreciation)	6,117,000		19,304,000
Selling and distribution expenses	$ 607,000		$ 1,403,000