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]
Revenues	$ 341,624		$ 320,099		$ 319,295		$ 312,165		$ 303,833		$ 298,549		$ 273,604		$ 273,904
Operating costs and expenses	320,499		355,132		298,055		289,417		298,616		278,540		253,965		255,981
Income before income taxes	21,619		(35,083)		22,043		23,129		6,481		21,193		19,858		18,957		31,708	66,489	70,264
Net income	$ 15,472		$ (22,182)		$ 13,921		$ 14,639		$ 5,452		$ 13,790		$ 12,933		$ 14,954		$ 21,850	$ 47,129	$ 44,214
Basic earnings per common share (in dollars per share)	$ 0.22	[1]	$ (0.31)	[1]	$ 0.20	[1]	$ 0.21	[1]	$ 0.08	[1]	$ 0.20	[1]	$ 0.19	[1]	$ 0.22	[1]	$ 0.31	$ 0.68	$ 0.65
Diluted earnings per common share (in dollars per share)	$ 0.22	[1]	$ (0.31)	[1]	$ 0.20	[1]	$ 0.21	[1]	$ 0.08	[1]	$ 0.20	[1]	$ 0.19	[1]	$ 0.22	[1]	$ 0.31	$ 0.67	$ 0.65
Cash dividends per common share (in dollars per share)	$ 0.175	[1]	$ 0.17375	[1]	$ 0.17250	[1]	$ 0.17125	[1]	$ 0.17	[1]	$ 0.16875	[1]	$ 0.1675	[1]	$ 0.16625	[1]

[1] Year-to-date earnings and cash dividends per common share amounts may differ from the sum of quarterly amounts due to rounding.