Earnings (Loss) 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
Numerator:
Net income (loss)	$ (36,168)	$ 7,695	$ 2,605	$ 3,006	$ (3,445)	$ 4,511	$ 3,876	$ 1,878	$ (22,862)	$ 6,820	$ 211,388
Denominator:
Weighted-average common shares outstanding, Basic (in shares)	109,737	109,555	110,060	111,464	110,788	110,214	109,904	109,733	110,198	110,162	114,814
Effect of potential dilutive common shares									0	2,978	2,670
Denominator:
Weighted-average common shares outstanding, Diluted (in shares)	109,737	113,119	112,565	115,325	110,788	113,723	112,061	112,252	110,198	113,140	117,484
Earnings Per Share, Basic									$ (0.21)	$ 0.06	$ 1.84
Earnings Per Share, Diluted									$ (0.21)	$ 0.06	$ 1.80