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 (loss)	$ (90,167)	$ (8,495)	$ 2,729	$ 40,310	$ 28,275	$ (8,040)	$ 2,019	$ 36,641	$ (55,623)	$ 58,895	$ 53,703
Average common shares outstanding - basic									28,669	27,647	27,347
Additional shares for stock-based compensation plans									0	132	70
Average common shares outstanding - diluted									28,669	27,779	27,417
Earnings (loss) per share of common stock - basic	$ (3.14)	$ (0.30)	$ 0.10	$ 1.41	$ 1.01	$ (0.29)	$ 0.07	$ 1.33	$ (1.94)	$ 2.13	$ 1.96
Earnings (loss) per share of common stock - diluted	$ (3.14)	$ (0.30)	$ 0.10	$ 1.40	$ 1.00	$ (0.29)	$ 0.07	$ 1.33	$ (1.94)	$ 2.12	$ 1.96
Antidilutive shares									97	5	12