Unaudited Quarterly Data Unaudited Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unaudited Quarterly Data [Abstract]
Total revenues	$ 661,750	$ 644,942	$ 655,730	$ 698,043	$ 638,954	$ 643,965	$ 677,635	$ 741,986	$ 2,660,465	$ 2,702,540	$ 2,963,252
Gross profit	419,564	407,740	423,886	436,978	410,881	412,465	432,533	462,663	1,688,168	1,718,542	1,935,049
Operating profit (loss)	13,624	25,632	27,151	(10,270)	(54,893)	(8,445)	(873)	1,152	56,137	(63,059)	(66,596)
Net earnings (loss)	$ 1,664	$ 12,918	$ 13,753	$ (19,843)	$ 34,824	$ (12,599)	$ (8,893)	$ (6,679)	$ 8,492	$ 6,653	$ (105,195)
Basic earnings (loss) per common share	$ 0.03	$ 0.24	$ 0.26	$ (0.37)	$ 0.65	$ (0.24)	$ (0.17)	$ (0.13)	$ 0.16	$ 0.12	$ (1.98)
Diluted earnings (loss) per common share	$ 0.03	$ 0.24	$ 0.25	$ (0.37)	0.65	(0.24)	(0.17)	(0.13)	0.16	0.12	(1.98)
Cash dividends declared per common share					$ 0.00	$ 0.00	$ 0.08	$ 0.08	$ 0.00	$ 0.16	$ 0.32