Activity for Real Estate Investments and Accumulated Depreciation (Details) - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate Investments [Abstract]
Balance at beginning of year		$ 8,069,395,000		$ 7,722,181,000
Acquisitions	[1]	316,151,000	$ 407,889,000	4,032,957,000
Less: FMV of Leases included in Acquisitions		(4,438,000)	(4,968,000)		$ (51,728,000)
Improvement and development		165,000,000	186,043,000	130,824,000
Disposition of real estate assets	[2]	(330,340,000)	(241,750,000)	(114,681,000)
Balance at end of year		8,215,768,000	8,069,395,000
Real Estate and Accumulated Depreciation Disclosure [Abstract]
Balance at beginning of year		1,373,678,000		1,138,315,000
Depreciation		289,177,000	276,991,000	165,885,000
Disposition of real estate assets	[2]	(163,642,000)	(41,628,000)	(61,879,000)
Balance at end of year		1,499,213,000	1,373,678,000
Commercial
Real Estate and Accumulated Depreciation Disclosure [Abstract]
Accumulated depreciation		16,845,000	15,279,000		14,108,000
Assets Held-for-sale
Real Estate Investments [Abstract]
Balance at beginning of year		0		$ (4,897,000)
Balance at end of year		0	0
Real Estate and Accumulated Depreciation Disclosure [Abstract]
Accumulated depreciation		$ 0	$ 0		$ (6,164,000)

[1] Includes non-cash activity related to acquisitions.

[2] Includes assets sold, casualty losses, and removal of certain fully depreciated assets.