Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 100,446	$ 102,140	$ 104,695	$ 85,474	$ 115,934	$ 102,996	$ 99,538	$ 90,429	$ 392,755	$ 408,897	$ 302,238
Total costs and expenses	59,630	62,411	62,097	60,046	76,079	65,521	63,818	59,035	244,184	264,453	202,697
Operating earnings	40,816	39,729	42,598	25,428	39,855	37,475	35,720	31,394	148,571	144,444	99,541
Net earnings	$ 33,129	$ 28,860	$ 32,727	$ 18,340	$ 25,893	$ 28,011	$ 30,737	$ 26,352	$ 113,056	$ 110,993	$ 57,284
Earnings per share, basic (in dollars per share)	$ 0.63	$ 0.55	$ 0.62	$ 0.35	$ 0.50	$ 0.54	$ 0.59	$ 0.51	$ 2.16	$ 2.13	$ 1.13
Earnings per share, diluted (in dollars per share)	$ 0.62	$ 0.54	$ 0.61	$ 0.34	$ 0.48	$ 0.52	$ 0.57	$ 0.49	$ 2.10	$ 2.05	$ 1.08