Summary Of Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data [Abstract]
Revenues	$ 58,061	$ 58,094	$ 57,190	$ 57,229	$ 57,253	$ 57,120	$ 57,005	$ 55,558	$ 230,574	$ 226,936	$ 207,915
Net income	$ 17,883	$ 20,299	$ 20,660	$ 21,667	$ 21,655	$ 21,036	$ 21,767	$ 22,019	$ 80,509	$ 86,477	$ 76,907
Net Income Per Common Share - Basic (in dollars per share)	$ 3.50	$ 3.97	$ 4.04	$ 4.24	$ 4.23	$ 4.11	$ 4.26	$ 4.31
Net Income Per Common Share - Diluted (in dollars per share)	$ 3.50	$ 3.97	$ 4.04	$ 4.24	$ 4.23	$ 4.11	$ 4.26	$ 4.31