Selected Quarterly Financial Data (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Data [Abstract]
Revenues	$ 250,239,000		$ 218,929,000		$ 211,507,000		$ 208,390,000		$ 202,087,000		$ 195,114,000		$ 192,954,000		$ 183,801,000
Operating costs and expenses	235,861,000		202,385,000		197,253,000		196,765,000		188,723,000		182,872,000		178,390,000		172,474,000
Income before income taxes	15,567,000		15,189,000		14,717,000		12,339,000		14,437,000		13,424,000		14,181,000		12,077,000		57,812,000	54,119,000	49,335,000
Net income	$ 10,565,000		$ 9,996,000		$ 9,828,000		$ 7,767,000		$ 9,123,000		$ 9,169,000		$ 8,721,000		$ 7,428,000		$ 38,156,000	$ 34,441,000	$ 30,342,000
Basic earnings per common share	$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.12	[1]	$ 0.14	[1]	$ 0.14	[1]	$ 0.13	[1]	$ 0.11	[1]	$ 0.57	$ 0.52	$ 0.46
Diluted earnings per common share, Per-share Amount	$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.12	[1]	$ 0.14	[1]	$ 0.14	[1]	$ 0.13	[1]	$ 0.11	[1]	$ 0.56	$ 0.51	$ 0.46
Cash dividends per common share	$ 0.1600		$ 0.1588		$ 0.1575		$ 0.15625		$ 0.16	[1]	$ 0.15	[1]	$ 0.15	[1]	$ 0.14	[1]	$ 0.63	$ 0.6	$ 0.49

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