Quarterly Financial Information (unaudited) (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 (unaudited)
Reduction in net revenues	$ 115,934	$ 102,996	$ 99,538	$ 90,429	$ 88,435	$ 80,398	$ 75,829	$ 57,576	$ 408,897	$ 302,238	$ 215,003
Total costs and expenses	76,079	65,521	63,818	59,035	54,091	58,056	49,762	40,788	264,453	202,697	160,787
Operating earnings	39,855	37,475	35,720	31,394	34,344	22,342	26,067	16,788	144,444	99,541	54,216
Net earnings	$ 25,893	$ 28,011	$ 30,737	$ 26,352	$ 13,658	$ 15,961	$ 17,368	$ 10,297	$ 110,993	$ 57,284	$ 91,221
Net earnings per share, basic	$ 0.50	$ 0.54	$ 0.59	$ 0.51	$ 0.27	$ 0.31	$ 0.34	$ 0.21	$ 2.13	$ 1.13	$ 1.84
Net earnings per share, diluted	$ 0.48	$ 0.52	$ 0.57	$ 0.49	$ 0.26	$ 0.29	$ 0.32	$ 0.19	$ 2.05	$ 1.08	$ 1.76