Summary Of Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of Quarterly Results (Unaudited)
Revenues	$ 57,253	$ 57,120	$ 57,005	$ 55,558	$ 52,819	$ 52,414	$ 50,646	$ 52,036	$ 226,936	$ 207,915	$ 200,814
Net income - basic and diluted	$ 21,655	$ 21,036	$ 21,767	$ 22,019	$ 23,572	$ 18,172	$ 17,341	$ 17,822	$ 86,477	$ 76,907	$ 67,925
Net Income Per Common Share - Basic (in dollars per share)	$ 4.23	$ 4.11	$ 4.26	$ 4.31	$ 4.61	$ 3.55	$ 3.39	$ 3.49
Net Income Per Common Share - Diluted (in dollars per share)	$ 4.23	$ 4.11	$ 4.26	$ 4.31	$ 4.61	$ 3.55	$ 3.39	$ 3.49