Significant Acquisitions and Dispositions Dispositions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) a sqft buildings entities	Dec. 31, 2014 USD ($) sqft buildings entities	Dec. 31, 2013 USD ($) sqft buildings
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment	$ 1,675,690	$ 493,217	$ 740,039
Mortgage notes receivable from buyers in property sales	$ 204,336	$ 0	$ 0
Number of Real Estate Properties Sold | buildings	153	29	38
Number of Unconsolidated Joint Ventures in which Equity Interests Held | entities	16
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital	$ 68,985	$ 91,750	$ 109,158
Earnings from Depreciated Property Sales Share of Joint Venture		$ 84,600	51,200
Medical Office [Member]
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment			$ 285,900
Number of Real Estate Properties Sold | buildings			18
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft			1,100,000
Unconsolidated Properties [Member]
Dispositions Disclosures [Line Items]
Number of Unconsolidated Joint Ventures in which Equity Interests Held | entities		6
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital			$ 92,300
Unconsolidated Properties [Member] | Industrial [Member]
Dispositions Disclosures [Line Items]
Number of Real Estate Properties Sold | buildings		11	1
Unconsolidated Properties [Member] | Office Building [Member]
Dispositions Disclosures [Line Items]
Number of Real Estate Properties Sold | buildings		11	19
Unconsolidated Properties [Member] | Retail [Member]
Dispositions Disclosures [Line Items]
Number of Real Estate Properties Sold | buildings		1
Atlanta Georgia [Member] | Unconsolidated Properties [Member] | Office Building [Member]
Dispositions Disclosures [Line Items]
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft		436,000
St. Louis Park, Minnesota [Member] | Unconsolidated Properties [Member] | Office Building [Member]
Dispositions Disclosures [Line Items]
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft		382,000
Cincinnati [Member]
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment		$ 150,500
Number of Real Estate Properties Sold | buildings		6
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft		1,000,000
South Florida [Member]
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment		$ 128,000
Number of Real Estate Properties Sold | buildings		2
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft		466,000
Industrial Portfolio Sale [Member]
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment	270,000
Gains (Losses) on Sales of Investment Real Estate	$ 107,400
Number of Real Estate Properties Sold | buildings	51
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft	5,200,000
Suburban Office Portfolio Sale [Member]
Dispositions Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment	$ 1,070,000
Gains (Losses) on Sales of Investment Real Estate	$ 406,100
Area of Land | a	57
Mortgage notes receivable from buyers in property sales	$ 200,000
Number of Real Estate Properties Sold | buildings	61
Square Footage Of Real Estate Properties Acquired Or Disposed | sqft	6,700,000
London Interbank Offered Rate (LIBOR) [Member] | Suburban Office Portfolio Sale [Member]
Dispositions Disclosures [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.50%