Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information [Line Items]
Gain (Loss) on Sale of Properties										$ 100,314	$ 91,778	$ 98,651
Operating revenue		$ 199,382	$ 198,972	$ 203,518	$ 206,901	$ 203,685	$ 198,356	$ 192,960	$ 197,630	808,773	792,631	645,930
Income from continuing operations		82,363	93,695	36,526	31,862	84,193	35,318	29,811	26,260	244,446	175,582	97,755
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		0	0	0	0	306	133	296	47,846	0	48,581	121,839
Net income		$ 82,363	$ 93,695	$ 36,526	$ 31,862	$ 84,499	$ 35,451	$ 30,107	$ 74,106	$ 244,446	$ 224,163	$ 219,594
Income per common share - basic	[1]	$ 0.55	$ 0.61	$ 0.24	$ 0.21	$ 0.56	$ 0.23	$ 0.20	$ 0.49
Income per common share - diluted	[1]	$ 0.55	$ 0.61	$ 0.24	$ 0.21	$ 0.55	$ 0.23	$ 0.20	$ 0.49

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