Selected Quarterly Financial Data - (Unaudited) (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 Information Disclosure [Abstract]
Total revenues	$ 185,402	$ 215,168	$ 197,664	$ 159,736	$ 168,831	$ 216,718	$ 190,930	$ 148,171	$ 757,970		$ 724,650		$ 692,728
Operating income	46,257	66,986	60,153	41,172	41,142	63,228	54,739	37,139	214,568		196,248		193,480
Income from continuing operations before income taxes	36,038	55,958	50,234	31,528	32,315	54,271	44,504	27,582	173,758		158,672		169,506
Income from continuing operations, net of income taxes	25,309	39,416	35,279	21,469	23,377	38,573	31,624	19,776	121,473		113,350		121,278
Income (loss) from discontinued operations, net of income taxes	(24)	(51)	121	1,641	66	143	183	(33)	1,687		359		335
Net income	$ 25,285	$ 39,365	$ 35,400	$ 23,110	$ 23,443	$ 38,716	$ 31,807	$ 19,743	$ 123,160		$ 113,709		$ 121,613
Per basic share:
Basic earnings per share, Continuing operations (in dollars per share)	$ 0.44	$ 0.67	$ 0.61	$ 0.37	$ 0.40	$ 0.66	$ 0.54	$ 0.34	$ 2.08	[1]	$ 1.94	[1]	$ 2.09
Basic earnings per share, Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.03	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.03	[1]	$ 0.00	[1]	$ 0.00
Per diluted share:
Diluted earnings per share, Continuing operations (in dollars per share)	$ 0.43	$ 0.67	$ 0.60	$ 0.36	$ 0.40	$ 0.65	$ 0.54	$ 0.34	$ 2.07	[1]	$ 1.92	[1]	$ 2.08
Diluted earnings per share, Discontinued Operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.03	$ 0.00	$ 0.01	$ 0.00	$ 0.00	$ 0.03	[1]	$ 0.01	[1]	$ 0.01

[1] The sum of the earnings per share for the four quarters may differ from annual earnings per share due to the required method of computing the weighted average shares in interim periods.