Unaudited Selected Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Revenue, Net	$ 171,852	$ 166,247	$ 163,903	$ 157,807	$ 149,542	$ 143,483	$ 148,069	$ 153,647	$ 659,809	$ 594,741	$ 797,520
Cost of Services and Goods Sold	121,660	120,890	117,118	114,572	112,678	107,153	109,999	112,814
Other operating expenses	17,814	17,881	17,378	20,056	15,340	14,842	17,843	17,893
Income Before Interest Expense and Taxes	32,378	27,476	29,407	23,179	21,524	21,488	20,227	22,940	112,440	86,179	161,070
Interest expense	2,717	2,707	2,692	2,618	2,548	2,569	3,021	3,434	10,734	11,572	12,380
Income before income tax expense	29,661	24,769	26,715	20,561	18,976	18,919	17,206	19,506	101,706	74,607	148,690
Income Tax Expense (Benefit)	8,009	3,716	4,006	2,879	3,607	2,081	671	4,389	18,610	10,748	33,758
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	21,652	21,053	22,709	17,682	15,369	16,838	16,535	15,117	83,096	63,859	114,932
Net Income (Loss) Attributable to Noncontrolling Interest	(39)	(33)	19	24	(90)	108	(89)	35	(29)	(36)	85
Net Income (Loss) Attributable to Parent	$ 21,691	$ 21,086	$ 22,690	$ 17,658	$ 15,459	$ 16,730	$ 16,624	$ 15,082	$ 83,125	$ 63,895	$ 114,847
Earnings Per Share, Basic	$ 0.49	$ 0.48	$ 0.51	$ 0.40	$ 0.35	$ 0.38	$ 0.38	$ 0.36	$ 1.88	$ 1.47	$ 2.69
Earnings Per Share, Diluted	$ 0.49	$ 0.48	$ 0.51	$ 0.40	$ 0.35	$ 0.38	$ 0.38	$ 0.35	$ 1.88	$ 1.46	$ 2.68
Weighted Average Number of Shares Outstanding, Basic	44,147	44,141	44,164	44,159	44,147	44,110	43,297	42,380	44,153	43,479	42,747
Weighted Average Number of Shares Outstanding, Diluted	44,276	44,332	44,374	44,347	44,326	44,320	43,505	42,520	44,264	43,670	42,908