Financial Results by Quarter (Unaudited) Financial Results by Quarter (Unaudited) (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
Quarterly Financial Data [Abstract]
Revenues	$ 175,244		$ 190,441		$ 163,229		$ 149,681		$ 155,681		$ 174,027		$ 141,495		$ 127,896		$ 678,595	$ 599,099	$ 575,336
Operating income (loss)	37,048		43,793		40,773		23,909		25,716		38,994		(34,228)		5,072		145,523	35,554	58,907
Net income (loss)	$ 11,588		$ 33,700		$ 24,244		$ 16,921		$ 14,373		$ 27,646		$ (31,238)		$ 157		$ 86,453	$ 10,938	$ 31,714
Net income (loss) per share
Basic (in dollars 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 (in dollars 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

[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.