Operations and Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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 [Abstract]
Net income	$ 46,696	$ 7,197	$ 6,139	$ (4,545)	$ 6,708	$ 7,337	$ 2,884	$ (2,265)	$ 55,487	$ 14,664	$ 30,498
Basic-weighted average shares outstanding									27,939	27,804	27,653
Effect of dilutive potential securities									232	160	205
Diluted-weighted average shares outstanding									28,171	27,964	27,858
Basic EPS (in dollars per share)	$ 1.67	$ 0.26	$ 0.22	$ (0.16)	$ 0.24	$ 0.26	$ 0.10	$ (0.08)	$ 1.99	$ 0.53	$ 1.10
Diluted EPS (in dollars per share)	$ 1.65	$ 0.26	$ 0.22	$ (0.16)	$ 0.24	$ 0.26	$ 0.10	$ (0.08)	$ 1.97	$ 0.52	$ 1.09
Shares excluded from computation of earnings per share									1,200	1,400	500