Quarterly Results of Operations (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 Results of Operations
Total revenue	$ 223,899	$ 215,273	$ 216,002	$ 222,579	$ 202,901	$ 215,907	$ 207,970	$ 209,719	$ 877,753	$ 836,497	$ 839,113
Net income	$ 28,102	$ 15,619	$ 25,934	$ 29,946	$ 18,346	$ 28,908	$ 26,816	$ 9,125	$ 99,601	$ 83,195	$ 62,387
Earnings per weighted average common share:
Basic (in dollars per share)	$ 1.84	$ 1.02	$ 1.70	$ 1.97	$ 1.21	$ 1.90	$ 1.77	$ 0.60	$ 6.52	$ 5.48	$ 4.13
Diluted (in dollars per share)	1.82	1.01	1.68	1.95	1.19	1.88	1.75	0.60	6.46	5.43	4.10
Cash dividends paid per common share (in dollars per share)	$ 0.90	$ 0.90	$ 0.80	$ 0.80	$ 0.80	$ 0.80	$ 0.80	$ 0.80	$ 3.40	$ 3.20	$ 3.00