Selected Quarterly Financial Data (Unaudited) (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 Information Disclosure [Abstract]
Revenues	$ 303,833		$ 298,549		$ 273,604		$ 273,904		$ 277,039		$ 272,681		$ 267,108		$ 260,607
Operating costs and expenses	298,616		278,540		253,965		255,981		257,814		255,070		248,730		248,477
Income before income taxes	6,481		21,193		19,858		18,957		19,625		18,573		18,283		13,783		66,489	70,264	57,812
Net income	$ 5,452		$ 13,790		$ 12,933		$ 14,954		$ 12,798		$ 11,517		$ 11,320		$ 8,579		$ 47,129	$ 44,214	$ 38,156
Basic earnings per common share (in dollars per share)	$ 0.08	[1]	$ 0.20	[1]	$ 0.19	[1]	$ 0.22	[1]	$ 0.19	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.13	[1]	$ 0.68	$ 0.65	$ 0.57
Diluted earnings per common share (in dollars per share)	$ 0.08	[1]	$ 0.20	[1]	$ 0.19	[1]	$ 0.22	[1]	$ 0.19	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.13	[1]	$ 0.67	$ 0.65	$ 0.56
Cash dividends per common share (in dollars per share)	$ 0.17	[1]	$ 0.16875	[1]	$ 0.1675	[1]	$ 0.16625	[1]	$ 0.165	[1]	$ 0.16375	[1]	$ 0.1625	[1]	$ 0.16125	[1]

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