Quarterly Financial Data (Schedule Of Quarterly Financial Information) (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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Abstract]
Revenues	$ 657,050	$ 640,688	$ 660,591	$ 608,136	$ 666,036	$ 610,929	$ 619,611	$ 557,903	$ 620,043	$ 586,534	$ 580,513	$ 535,195	$ 2,566,465	$ 2,454,479	$ 2,322,285
Gross profit	55,199	52,777	54,299	58,318	60,661	58,243	59,484	47,532	58,896	54,180	53,200	48,409	220,593	225,920	214,685
Net income (loss)	$ 27,716	$ 7,799	$ 10,943	$ (23,641)	$ (76,361)	$ 17,831	$ 18,074	$ 8,555	$ 19,327	$ 22,544	$ 12,688	$ 9,374	$ 22,817	$ (31,901)	$ 63,933
Earnings per common share, Basic	$ 0.64	$ 0.17	$ 0.23	$ (0.49)	$ (1.54)	$ 0.36	$ 0.36	$ 0.17	$ 0.39	$ 0.46	$ 0.26	$ 0.19	$ 0.49	$ (0.64)	$ 1.3
Earnings per common share, Diluted	$ 0.64	$ 0.17	$ 0.23	$ (0.49)	$ (1.54)	$ 0.35	$ 0.36	$ 0.17	$ 0.39	$ 0.46	$ 0.26	$ 0.19	$ 0.49	$ (0.64)	$ 1.28