Unaudited Quarterly Data Unaudited Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unaudited Quarterly Data [Abstract]
Total revenues	$ 667,862	$ 649,371	$ 655,925	$ 696,694	$ 661,750	$ 644,942	$ 655,730	$ 698,043	$ 2,669,852	$ 2,660,465	$ 2,702,540
Gross profit	411,138	399,996	408,071	424,866	419,564	407,740	423,886	436,978	1,644,071	1,688,168	1,718,542
Operating profit (loss)	67,834	38,847	129,829	17,349	13,624	25,632	27,151	(10,270)	253,859	56,137	(63,059)
Net earnings	$ 40,491	$ 31,277	$ 94,455	$ 7,323	$ 1,664	$ 12,918	$ 13,753	$ (19,843)	$ 173,546	$ 8,492	$ 6,653
Basic earnings per common share	$ 0.74	$ 0.57	$ 1.74	$ 0.14	$ 0.03	$ 0.24	$ 0.26	$ (0.37)	$ 3.19	$ 0.16	$ 0.12
Diluted earnings per common share	$ 0.72	$ 0.56	$ 1.70	$ 0.13	$ 0.03	$ 0.24	$ 0.25	$ (0.37)	$ 3.10	$ 0.16	$ 0.12