Quarterly Results of Operations (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 [Line Items]
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax													$ 50,200				$ 11,383	$ 60,582	$ 6,857
Operating revenue	176,227		171,727		168,567		169,031		168,704		165,192		163,948	[1]	165,397		685,552	663,241	652,009
Income from continuing operations	32,178		31,742		34,460		35,450		34,387		35,889		38,987	[1]	30,798		133,830	140,061	130,140
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	8,392		(1,845)		3,221		4,153		6,858		5,792		53,854	[1]	4,145		13,921	70,649	23,235
Net income	$ 40,570		$ 29,897		$ 37,681		$ 39,603		$ 41,245		$ 41,681		$ 92,841	[1]	$ 34,943		$ 147,751	$ 210,710	$ 153,375
Earnings Per Share, Basic	$ 0.33	[2]	$ 0.24	[2]	$ 0.29	[2]	$ 0.32	[2]	$ 0.30	[2]	$ 0.31	[2]	$ 0.74	[1],[2]	$ 0.25	[2]	$ 1.18	$ 1.60	$ 1.13
Earnings Per Share, Diluted	$ 0.32	[2]	$ 0.24	[2]	$ 0.29	[2]	$ 0.32	[2]	$ 0.30	[2]	$ 0.31	[2]	$ 0.74	[1],[2]	$ 0.25	[2]	$ 1.17	$ 1.59	$ 1.12

[1] Includes gain on sale included in discontinued operations of $50.2 million.

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