Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Income	$ 28,275	$ (8,040)	$ 2,019	$ 36,641	$ 29,705	$ (6,685)	$ 2,197	$ 28,486	$ 58,895	$ 53,703	$ 58,692
Average common shares outstanding - basic									27,647	27,347	27,164
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements									132	70	59
Average common shares outstanding - diluted									27,779	27,417	27,223
Earnings per share of common stock - basic	$ 1.01	$ (0.29)	$ 0.07	$ 1.33	$ 1.08	$ (0.24)	$ 0.08	$ 1.04	$ 2.13	$ 1.96	$ 2.16
Earnings per share of common stock - diluted	$ 1.00	$ (0.29)	$ 0.07	$ 1.33	$ 1.08	$ (0.24)	$ 0.08	$ 1.04	$ 2.12	$ 1.96	$ 2.16
Antidilutive shares excluded from computation of earnings per share amount									5	12	18