Investments Summary of Assets, Liabilities and Results of Operations for Significant Unconsolidated Joint Ventures (Details) - Significant Unconsolidated Joint Ventures - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Current assets	$ 503,043	$ 383,203
Noncurrent assets	1,041,519	913,643
Total assets	1,544,562	1,296,846
Current liabilities	133,670	138,474
Noncurrent liabilities	405,662	319,801
Total liabilities	539,332	458,275
Net sales	687,561	590,980	$ 560,376
Gross profit	353,577	267,241	253,569
Income before income taxes	267,805	189,016	157,501
Net income	$ 184,777	$ 126,872	$ 111,491