Quarterly Results of Operations (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 [Line Items]
Operating revenue	$ 169,801		$ 166,247		$ 165,027		$ 166,519		$ 164,530		$ 166,316		$ 161,789		$ 164,366		$ 667,594	$ 657,001	$ 644,696
Income from continuing operations	34,316		35,857		38,934		30,764		32,062		35,900		33,576		28,464		139,871	130,002	30,526
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	6,929		5,824		53,907		4,179		4,471		5,106		6,438		7,358		70,839	23,373	48,466
Net income	$ 41,245		$ 41,681		$ 92,841		$ 34,943		$ 36,533		$ 41,006		$ 40,014		$ 35,822		$ 210,710	$ 153,375	$ 78,992
Earnings Per Share, Basic	$ 0.30	[1]	$ 0.31	[1]	$ 0.74	[1]	$ 0.25	[1]	$ 0.27	[1]	$ 0.31	[1]	$ 0.30	[1]	$ 0.26	[1]	$ 1.60	$ 1.13	$ 0.52
Earnings Per Share, Diluted	$ 0.30	[1]	$ 0.31	[1]	$ 0.74	[1]	$ 0.25	[1]	$ 0.26	[1]	$ 0.30	[1]	$ 0.29	[1]	$ 0.26	[1]	$ 1.59	$ 1.12	$ 0.52

[1] The sum of quarterly financial data may vary from the annual data due to rounding.