Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 277,039		$ 272,681		$ 267,108		$ 260,607		$ 250,239		$ 218,929		$ 211,507		$ 208,390
Operating costs and expenses	257,814		255,070		248,730		248,477		235,861		202,385		197,253		196,765
Income before income taxes	19,625		18,573		18,283		13,783		15,567		15,189		14,717		12,339		70,264	57,812	54,119
Net income	$ 12,798		$ 11,517		$ 11,320		$ 8,579		$ 10,565		$ 9,996		$ 9,828		$ 7,767		$ 44,214	$ 38,156	$ 34,441
Basic earnings per common share (dollars per share)	$ 0.19	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.13	[1]	$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.12	[1]	$ 0.65	$ 0.57	$ 0.52
Diluted earnings per common share (dollars per share)	$ 0.19	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.13	[1]	$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.12	[1]	$ 0.65	$ 0.56	$ 0.51
Cash dividends per common share (dollars per share)	$ 0.16500	[1]	$ 0.16375	[1]	$ 0.16250	[1]	$ 0.16125	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.65	$ 0.63	$ 0.60

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