Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Gain (Loss) on Sale of Properties	$ 43,200																$ 91,661	$ 98,403	$ 12,426
Operating revenue	203,685		198,356		192,960		197,630		192,427		155,349		151,026		147,128		792,631	645,930	560,279
Income from continuing operations	84,193	[1]	35,318		29,811		26,260		19,911		23,406		29,611		24,827		175,582	97,755	96,747
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	306		133		296		47,846		51,832		6,544		13,633		49,830		48,581	121,839	51,004
Net income	$ 84,499		$ 35,451		$ 30,107		$ 74,106		$ 71,743		$ 29,950		$ 43,244		$ 74,657		$ 224,163	$ 219,594	$ 147,751
Income per common share - basic	$ 0.56	[2]	$ 0.23	[2]	$ 0.20	[2]	$ 0.49	[2]	$ 0.48	[2]	$ 0.21	[2]	$ 0.33	[2]	$ 0.60	[2]
Income per common share - diluted	$ 0.55	[2]	$ 0.23	[2]	$ 0.20	[2]	$ 0.49	[2]	$ 0.48	[2]	$ 0.21	[2]	$ 0.33	[2]	$ 0.60	[2]

[1] Includes gain on sale of $43.2 million.

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