Earnings and Dividends Per Share (Summary Of Calculation Of Basic And Diluted Earnings Per Share) (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
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 37,083	$ 40,979	$ 37,839	$ 38,432	$ 37,798	$ 39,092	$ 38,245	$ 44,230	$ 154,333	$ 159,365	$ 151,540
BASIC:
Weighted Average Number of Shares Outstanding, Basic	42,351,443	42,187,077	41,704,819	41,532,154	41,532,130	41,108,699	40,982,244	39,953,804	41,943,873	40,894,219	39,013,412
Net income per common share attributable to common stockholders	$ 0.88	$ 0.97	$ 0.91	$ 0.93	$ 0.91	$ 0.95	$ 0.93	$ 1.11	$ 3.68	$ 3.90	$ 3.88
DILUTED:
Weighted average common shares outstanding, diluted									41,943,873	40,894,219	39,013,412
Stock options, diluted									67,735	67,703	52,497
Convertible subordinated debentures									80,123	189,531	89,471
Average dilutive common shares outstanding	42,568,720	42,434,499	41,786,829	41,576,876	41,803,615	41,448,263	41,245,173	40,108,762	42,091,731	41,151,453	39,155,380
Net income per common share attributable to common stockholders	$ 0.87	$ 0.97	$ 0.91	$ 0.92	$ 0.90	$ 0.94	$ 0.93	$ 1.10	$ 3.67	$ 3.87	$ 3.87
Incremental shares excluded since anti-dilutive:
Stock options with an exercise price in excess of the market price for our common shares									518	573	6,366
Common Stock, Dividends, Per Share, Declared, Regular									$ 4.00	$ 3.80	$ 3.60