Financial Results by Quarter Financial Results by Quarter (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Data [Abstract]
Revenues	$ 146,237		$ 177,215		$ 143,919		$ 139,579		$ 139,955		$ 157,869		$ 133,212		$ 139,253		$ 606,950	$ 570,289	$ 525,134
Operating Income	26,603		44,869		29,725		31,309		20,645		30,904		29,441		26,608		132,506	107,598	84,942
Net income (loss)	$ 20,136		$ 33,154		$ 16,270		$ 20,350		$ 13,721		$ 22,191		$ 19,182		$ 15,487		$ 89,910	$ 70,581	$ 42,594
Basic (in dollars per share)	$ 0.49	[1]	$ 0.81	[1]	$ 0.40	[1]	$ 0.50	[1]	$ 0.34	[1]	$ 0.55	[1]	$ 0.47	[1]	$ 0.38	[1]	$ 2.21	$ 1.74	$ 1.06
Diluted (in dollars per share)	$ 0.49	[1]	$ 0.81	[1]	$ 0.40	[1]	$ 0.50	[1]	$ 0.34	[1]	$ 0.54	[1]	$ 0.47	[1]	$ 0.38	[1]	$ 2.20	$ 1.73	$ 1.05

[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.