Unaudited Quarterly Data Unaudited Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unaudited Quarterly Data [Abstract]
Revenues	$ 638,954	$ 643,965	$ 677,635	$ 741,986	$ 684,104	$ 693,877	$ 749,619	$ 835,652	$ 2,702,540	$ 2,963,252	$ 3,278,420
Gross profit	410,881	412,465	432,533	462,663	442,721	457,226	500,158	534,944	1,718,542	1,935,049	2,118,109
Operating profit (Loss)	(54,893)	(8,445)	(873)	1,152	(159,276)	16,700	27,550	48,430	(63,059)	(66,596)	(1,007,888)
Net earnings (loss)	$ 34,824	$ (12,599)	$ (8,893)	$ (6,679)	$ (146,383)	$ 6,181	$ 9,946	$ 25,061	$ 6,653	$ (105,195)	$ (953,520)
Basic earnings (loss) per common share	$ 0.65	$ (0.24)	$ (0.17)	$ (0.13)	$ (2.76)	$ 0.12	$ 0.19	$ 0.47	$ 0.12	$ (1.98)	$ (17.97)
Diluted earnings (loss) per common share	0.65	(0.24)	(0.17)	(0.13)	(2.76)	0.12	0.19	0.47	0.12	(1.98)	(17.97)
Cash dividends declared per common share	$ 0.00	$ 0.00	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.16	$ 0.32	$ 0.96