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	$ 366,082	$ 360,165	$ 355,356	$ 355,246	$ 341,624	$ 320,099	$ 319,295	$ 312,165
Operating costs and expenses	355,667	332,090	329,341	330,699	320,499	355,132	298,055	289,417
Income before income taxes	11,712	26,741	26,257	25,054	21,619	(35,083)	22,043	23,129	$ 89,764	$ 31,708	$ 66,489
Net income	$ 9,134	$ 17,086	$ 16,288	$ 15,516	$ 15,472	$ (22,182)	$ 13,921	$ 14,639	$ 58,024	$ 21,850	$ 47,129
Basic earnings per common share (in dollars per share)	$ 0.13	$ 0.24	$ 0.23	$ 0.22	$ 0.22	$ (0.31)	$ 0.20	$ 0.21	$ 0.81	$ 0.31	$ 0.68
Diluted earnings per common share (in dollars per share)	0.13	0.24	0.23	0.22	0.22	(0.31)	0.20	0.21	$ 0.80	$ 0.31	$ 0.67
Cash dividends per common share (in dollars per share)	$ 0.18000	$ 0.17875	$ 0.1775	$ 0.17625	$ 0.175	$ 0.17375	$ 0.17250	$ 0.17125