Mortgage Notes Payable (Schedule of Mortgage Notes Payable) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Nov. 05, 2013
Debt Instrument [Line Items]
Outstanding Loan Amount	$ 3,759,935,000		$ 1,301,114,000
Loss on derivative instruments, net	(10,570,000)		(67,946,000)		0
Gain from write-off of deferred financing costs and net premiums	(14,012,000)		0		0
CapLease [Member]
Debt Instrument [Line Items]
Building, historical cost						40,000,000
Mortgages [Member]
Debt Instrument [Line Items]
Encumbered Properties	776		177
Outstanding Loan Amount	3,689,795,000		1,258,661,000
Weighted Average Effective Interest Rate	4.88%	[1]	3.42%	[1]
Unamortized discount (premium), net	(70,100,000)
Extinguishment of debt, amount	1,600,000,000
Payments for prepayments fees	35,900,000
Gain from write-off of deferred financing costs and net premiums	(18,300,000)
Mortgages [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Weighted Average Effective Interest Rate	4.72%	[2]
Mortgages [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Loss on derivative instruments, net	11,400,000
Mortgages [Member] | CapLease [Member]
Debt Instrument [Line Items]
Unamortized discount (premium), net	(137,400,000)
Mortgages [Member] | Weighted Average [Member]
Debt Instrument [Line Items]
Weighted Average Maturity	6 years 1 month 35 days	[2]	3 years 4 months 28 days	[2]
Mortgages [Member] | Weighted Average [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Weighted Average Maturity	4 years 0 months 21 days
Mortgages [Member] | Minimum [Member]
Debt Instrument [Line Items]
Effective interest rate	2.75%		1.83%
Mortgages [Member] | Maximum [Member]
Debt Instrument [Line Items]
Effective interest rate	7.20%		6.28%
Mortgages [Member] | NIH Loan [Member] | CapLease [Member]
Debt Instrument [Line Items]
Outstanding Loan Amount	$ 53,800,000

[1] Mortgage notes payable primarily have fixed rates or are fixed by way of interest rate swap arrangements. Effective interest rates range from 2.75% to 7.20% at December 31, 2014 and 1.83% to 6.28% at December 31, 2013.

[2] Weighted-average remaining years until maturity as of December 31, 2014 and December 31, 2013, respectively.