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, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Revenues	$ 137,989		$ 174,475		$ 128,747		$ 134,125		$ 146,237		$ 177,215		$ 143,919		$ 139,579		$ 575,336	$ 606,950	$ 570,289
Operating Income	9,855		27,714		7,311		14,027		26,603		44,869		29,725		31,309		58,907	132,506	107,598
Net income	$ 3,549		$ 21,798		$ 711		$ 5,656		$ 20,136		$ 33,154		$ 16,270		$ 20,350		$ 31,714	$ 89,910	$ 70,581
Basic (in dollars per share)	$ 0.09	[1]	$ 0.53	[1]	$ 0.02	[1]	$ 0.14	[1]	$ 0.49	[1]	$ 0.81	[1]	$ 0.40	[1]	$ 0.50	[1]	$ 0.78	$ 2.21	$ 1.74
Diluted (in dollars per share)	$ 0.09	[1]	$ 0.53	[1]	$ 0.02	[1]	$ 0.14	[1]	$ 0.49	[1]	$ 0.81	[1]	$ 0.40	[1]	$ 0.50	[1]	$ 0.77	$ 2.20	$ 1.73

[1] 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.