Selected Quarterly Financial Data (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 Information Disclosure [Abstract]
Revenues	$ 171,300	$ 162,437	$ 171,549	$ 175,814	$ 164,413	$ 162,853	$ 160,521	$ 167,201	$ 681,100	$ 654,988	$ 640,467
Operating Margin	105,228	96,685	96,807	100,286	91,941	91,629	94,473	97,072	399,006	375,115	369,692
Net income	$ 25,357	$ 24,257	$ 26,897	$ 28,412	$ (22,425)	$ 18,887	$ 19,167	$ 20,824	$ 104,923	$ 36,453	$ 72,037
Basic (USD per share)	$ 1.36	$ 1.29	$ 1.41	$ 1.49	$ (1.18)	$ 0.99	$ 1.00	$ 1.09	$ 5.55	$ 1.92	$ 3.75
Diluted (USD per share)		$ 1.28	$ 1.40	$ 1.47	$ 1.35	$ 0.99	$ 1.00	$ 1.08	$ 5.52	$ 1.90	$ 3.70