Accounting for the Impairment or Disposal of Long-Lived Assets (Significant Sale) (Details) $ in Thousands, ft² in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2016 USD ($) a ft² properties	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Real Estate [Line Items]
Net income		$ 200,291	$ 55,734	$ 51,057	$ 59,063	$ 83,841	$ 93,695	$ 36,526	$ 31,862	$ 366,145	$ 245,924	$ 224,471
Noncontrolling interest – operating partnership										(9,070)	(6,192)	(5,713)
Income available to common shareholders										356,817	239,483	218,211
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Real Estate Portfolio [Member]
Real Estate [Line Items]
Number of properties | properties	108
Leaseable square feet | ft²	7.6
Area of land | a	26.7
Proceeds from sale of land	$ 969,000
Gains on sale of land	$ 179,100
Net income										48,609	58,792	51,898
Noncontrolling interest – operating partnership										(1,142)	(1,370)	(1,220)
Income available to common shareholders										$ 47,467	$ 57,422	$ 50,678