Quarterly Results (unaudited) (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, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 221,218	$ 209,532	$ 213,672	$ 204,184	$ 217,402	$ 194,424	$ 191,589	$ 185,072
Gross profit	117,922	114,295	118,500	116,147	114,980	107,419	104,594	99,590	$ 466,864	$ 426,583	$ 392,422
Income from operations	14,679	15,783	11,374	17,581	19,959	18,423	16,523	13,273	59,417	68,178	68,224
Income before provision for income taxes	11,485	11,496	7,417	14,224	17,226	15,799	14,134	11,182	44,622	58,341	57,350
Net income	$ 9,334	$ 11,164	$ 6,592	$ 17,751	$ 36,638	$ 12,824	$ 11,029	$ 13,142	$ 44,841	$ 73,633	$ 45,404
Earnings per share, basic	$ 0.20	$ 0.24	$ 0.14	$ 0.38	$ 0.78	$ 0.27	$ 0.24	$ 0.28	$ 0.95	$ 1.58	$ 0.98
Earnings per share, diluted	$ 0.19	$ 0.23	$ 0.14	$ 0.37	$ 0.76	$ 0.27	$ 0.23	$ 0.28	$ 0.93	$ 1.54	$ 0.96