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, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Data [Abstract]
Revenues	$ 139,955		$ 157,869		$ 133,212		$ 139,253		$ 143,299		$ 151,911		$ 117,540		$ 112,384		$ 570,289	$ 525,134	$ 497,421
Operating Income	20,645		30,904		29,441		26,608		26,570		28,763		17,090		12,519		107,598	84,942	72,240
Net income (loss)	$ 13,721		$ 22,191		$ 19,182		$ 15,487		$ 13,864		$ 18,599		$ 5,080		$ 5,051		$ 70,581	$ 42,594	$ 40,266
Basic (in dollars per share)	$ 0.34	[1]	$ 0.55	[1]	$ 0.47	[1]	$ 0.38	[1]	$ 0.34	[1]	$ 0.46	[1]	$ 0.13	[1]	$ 0.13	[1]	$ 1.74	$ 1.06	$ 1.00
Diluted (in dollars per share)	$ 0.34	[1]	$ 0.54	[1]	$ 0.47	[1]	$ 0.38	[1]	$ 0.34	[1]	$ 0.46	[1]	$ 0.13	[1]	$ 0.13	[1]	$ 1.73	$ 1.05	$ 1.00

[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.