Common Stock (Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Basic number of common shares outstanding	40,517,713	40,409,030	40,504,526	40,398,192	40,467,231	40,381,776
Dilutive effect of unvested performance awards	130,086	116,555	113,519	101,152	126,818	85,219
Diluted number of common shares outstanding	40,647,799	40,525,585	40,618,045	40,499,344	40,594,049	40,466,995
Net income	$ 33,295	$ 36,066	$ 26,329	$ 32,077	$ 92,513	$ 112,369
Income allocated to participating restricted stock	(124)	(142)	(98)	(119)	(336)	(423)
Net income available to common shareholders, basic	33,171	35,924	26,231	31,958	92,177	111,946
Income reallocated to participating restricted stock	(124)	(142)	(98)	(119)	(336)	(423)
Net income available to common shareholders, diluted	$ 33,171	$ 35,924	$ 26,231	$ 31,958	$ 92,177	$ 111,946
Earnings Per Share, Basic, Distributed	$ 0.36	$ 0.335	$ 0.695	$ 0.645	$ 1.365	$ 1.265
Earnings (losses) Per Share, Basic, Undistributed	0.46	0.555	(0.045)	0.145	0.915	1.505
Basic net income per common share	0.82	0.890	0.650	0.790	2.280	2.770
Earnings Per Share, Diluted, Distributed	0.36	0.335	0.695	0.645	1.365	1.265
Earnings (losses) Per Share, Diluted, Undistributed	0.46	0.555	(0.045)	0.145	0.905	1.505
Diluted net income per common share	$ 0.82	$ 0.890	$ 0.650	$ 0.790	$ 2.270	$ 2.770