Business Combinations (Pro Forma) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Effect of Exchange Rate on Revenue	$ 9,800		$ 6,100
Business Acquisition, Pro Forma Information
Operating revenue	111,312	$ 144,275	249,188	$ 287,199
Net Income (Loss) Attributable to Parent	$ 23,475	$ 28,851	$ 48,198	$ 54,561
Basic EPS	$ 0.77	$ 0.95	$ 1.58	$ 1.79
Diluted EPS	$ 0.76	$ 0.94	$ 1.57	$ 1.78
Operating revenue	$ 111,312	$ 144,275	$ 249,188	$ 287,199
Parent, Zillious, Essel, and Wallstreet [Member]
Business Acquisition, Pro Forma Information
Revenue, pro forma	112,018	148,169	253,766	297,334
Net income, pro forma	$ 22,354	$ 24,233	$ 42,128	$ 43,791
Basic EPS, pro forma	$ 0.73	$ 0.79	$ 1.38	$ 1.43
Diluted EPS, pro forma	$ 0.73	$ 0.79	$ 1.37	$ 1.43