Earnings per Share (Reconciliation Of Number Of Common Shares Used In Calculating Basic And Diluted EPS) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings per Share [Line Items]
Net income (loss)	$ 24,244	$ (31,238)	$ 41,165	$ (31,081)
Basic weighted-average shares outstanding	40,822,726	40,784,129	40,802,057	40,836,503
Diluted weighted-average shares outstanding	41,218,969	40,784,129	41,143,695	40,836,503
Basic net income (loss) per share	$ 0.59	$ (0.77)	$ 1.01	$ (0.76)
Diluted net income (loss) per share	$ 0.59	$ (0.77)	$ 1.00	$ (0.76)
Performance shares [Member]
Earnings per Share [Line Items]
Incremental shares	355,844		305,625
Restricted stock units [Member]
Earnings per Share [Line Items]
Incremental shares	40,399		36,013