Quarterly Financial Data (Schedule Of Quarterly Financial Information) (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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Revenues	$ 508,444	$ 555,229	$ 601,602	$ 602,820	$ 657,050	$ 640,688	$ 660,591	$ 608,136	$ 666,036	$ 610,929	$ 619,611	$ 557,903	$ 2,268,095	$ 2,566,465	$ 2,454,479
Gross profit	40,725	52,883	52,998	53,800	55,199	52,777	54,299	58,318	60,661	58,243	59,484	47,532	200,406	220,593	225,920
Net income (loss)	$ (6,931)	$ 7,136	$ 9,447	$ 13,773	$ 27,716	$ 7,799	$ 10,943	$ (23,641)	$ (76,361)	$ 17,831	$ 18,074	$ 8,555	$ 23,425	$ 22,817	$ (31,901)
Earnings per common share, Basic	$ (0.19)	$ 0.19	$ 0.25	$ 0.34	$ 0.64	$ 0.17	$ 0.23	$ (0.49)	$ (1.54)	$ 0.36	$ 0.36	$ 0.17	$ 0.61	$ 0.49	$ (0.64)
Earnings per common share, Diluted	$ (0.19)	$ 0.19	$ 0.24	$ 0.34	$ 0.64	$ 0.17	$ 0.23	$ (0.49)	$ (1.54)	$ 0.35	$ 0.36	$ 0.17	$ 0.60	$ 0.49	$ (0.64)