Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	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
Quarterly Financial Information (Unaudited) [Abstract]
Operating Revenue	$ 131,210	$ 132,529	$ 137,192	$ 127,692	$ 129,243	$ 127,245	$ 127,411	$ 115,617	$ 528,623	$ 499,516	$ 459,539
Operating Income (Loss)	26,865	25,132	32,687	21,873	24,524	29,061	22,033	15,831	106,557	91,449	78,964
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	27,019	25,306	32,895	22,110	24,782	29,408	22,449	16,234	107,330	92,873	81,302
Net income	$ 17,122	$ 15,399	$ 22,532	$ 14,879	$ 15,379	$ 18,297	$ 16,653	$ 11,887	$ 69,932	$ 62,216	$ 56,949
Basic EPS	$ 0.20	$ 0.17	$ 0.25	$ 0.16	$ 0.17	$ 0.20	$ 0.18	$ 0.13	$ 0.78	$ 0.69	$ 0.62