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																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings per Share [Line Items]
Net income	$ 11,588		$ 33,700		$ 24,244		$ 16,921		$ 14,373		$ 27,646		$ (31,238)		$ 157		$ 86,453	$ 10,938	$ 31,714
Basic weighted-average shares outstanding																	40,824,420	40,797,806	40,842,126
Diluted weighted-average shares outstanding																	41,227,388	41,033,440	40,987,819
Basic net income per share	$ 0.28	[1]	$ 0.83	[1]	$ 0.59	[1]	$ 0.41	[1]	$ 0.35	[1]	$ 0.68	[1]	$ (0.77)	[1]	$ 0	[1]	$ 2.12	$ 0.27	$ 0.78
Diluted net income per share	$ 0.28	[1]	$ 0.82	[1]	$ 0.59	[1]	$ 0.41	[1]	$ 0.35	[1]	$ 0.68	[1]	$ (0.77)	[1]	$ 0	[1]	$ 2.10	$ 0.27	$ 0.77
Performance shares [Member]
Earnings per Share [Line Items]
Incremental shares																	362,509	200,164	122,334
Restricted stock units [Member]
Earnings per Share [Line Items]
Incremental shares																	40,459	35,470	23,359

[1] Per share amounts are computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the total computed for the year.