Common Stock (Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Basic number of common shares outstanding	40,363,819	40,289,010	40,344,834	40,267,533	40,332,835	40,255,439
Dilutive effect of unvested performance awards	62,123	40,519	50,977	32,268	47,608	24,201
Diluted number of common shares outstanding	40,425,942	40,329,529	40,395,811	40,299,801	40,380,443	40,279,640
Net income	$ 74,636	$ 56,740	$ 91,112	$ 81,270	$ 91,760	$ 85,511
Income allocated to participating restricted stock	(232)	(184)	(271)	(253)	(264)	(265)
Net income available to common shareholders, basic	74,404	56,556	90,841	81,017	91,496	85,246
Income reallocated to participating restricted stock	(232)	(184)	(271)	(253)	(264)	(265)
Net income available to common shareholders, diluted	$ 74,404	$ 56,556	$ 90,841	$ 81,017	$ 91,496	$ 85,246
Earnings Per Share, Basic, Distributed	$ 0.310	$ 0.295	$ 0.915	$ 0.870	$ 1.21	$ 1.15
Earnings Per Share, Basic, Undistributed	1.530	1.105	1.335	1.140	1.06	0.97
Basic net income per common share	1.840	1.400	2.250	2.010	2.270	2.120
Earnings Per Share, Diluted, Distributed	0.310	0.295	0.915	0.870	1.21	1.15
Earnings Per Share, Diluted, Undistributed	1.530	1.105	1.335	1.140	1.06	0.97
Diluted net income per common share	$ 1.840	$ 1.400	$ 2.250	$ 2.010	$ 2.270	$ 2.120