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 ($) ft² a properties	Dec. 31, 2017 USD ($) a bldg	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) a bldg	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Real Estate [Line Items]
Number of properties | bldg		37								37
Area of land | a		9								9
Net income		$ 131,824	$ 61,159	$ 52,818	$ 44,244	$ 200,291	$ 55,734	$ 51,057	$ 59,063	$ 290,045	$ 366,145	$ 245,924
Noncontrolling interest – operating partnership										(7,224)	(9,070)	(6,192)
Income available to common shareholders										$ 282,340	356,817	239,483
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Real Estate [Line Items]
Number of properties | bldg		8								8
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 and building	$ 179,100
Net income											48,609	58,792
Noncontrolling interest – operating partnership											(1,142)	(1,370)
Income available to common shareholders											$ 47,467	$ 57,422