Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information [Line Items]
Operating revenue	$ 192,427		$ 155,349		$ 151,026		$ 147,128		$ 144,431		$ 140,515		$ 137,987		$ 137,346
Income from continuing operations	19,911		23,406		29,611		24,827		23,776		22,358		24,725		25,888		97,755	96,747	104,645
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	51,832		6,544		13,633		49,830		16,795		7,539		12,956		13,714		121,839	51,004	106,065
Net income	$ 71,743		$ 29,950		$ 43,244		$ 74,657		$ 40,571		$ 29,897		$ 37,681		$ 39,602		$ 219,594	$ 147,751	$ 210,710
Earnings Per Share, Basic	$ 0.48	[1]	$ 0.21	[1]	$ 0.33	[1]	$ 0.60	[1]	$ 0.33	[1]	$ 0.24	[1]	$ 0.29	[1]	$ 0.32	[1]	$ 1.61	$ 1.18	$ 1.60
Earnings Per Share, Diluted	$ 0.48	[1]	$ 0.21	[1]	$ 0.33	[1]	$ 0.60	[1]	$ 0.32	[1]	$ 0.24	[1]	$ 0.29	[1]	$ 0.32	[1]	$ 1.60	$ 1.17	$ 1.59

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