Notes and Bonds Payable (Details)						1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 26, 2015 USD ($) ft²	Jun. 01, 2015 USD ($) ft²	May. 15, 2015 USD ($)	May. 04, 2015 USD ($) ft²	Apr. 01, 2015 USD ($) ft²	Jun. 30, 2015 USD ($) ft²	Jan. 31, 2015 USD ($) ft²	Jun. 30, 2015 USD ($) ft²	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) ft² swap_agreement	Jun. 30, 2014 USD ($)	Apr. 24, 2015 USD ($)
Debt Instrument [Line Items]
Number of forward starting swaps | swap_agreement										4
Loss on early extinguishment of debt								$ (27,998,000)	$ 0	$ (27,998,000)	$ 0
Area of Real Estate Property | ft²						14,100,000		14,100,000		14,100,000
Washington [Member] | Real Estate Acquisitions [Member]
Debt Instrument [Line Items]
Stated interest rate (percent)	5.75%					5.75%		5.75%		5.75%
Effective interest rate (percent)	5.07%
Square footage | ft²	35,558					35,558		35,558		35,558
Mortgage notes payable assumed	$ 9,500,000					$ 9,500,000
Fair value premium recorded upon acquisition	$ 200,000					$ 200,000
Medical Office Building [Member]
Debt Instrument [Line Items]
Area of Real Estate Property | ft²						432,000,000		432,000,000		432,000,000
Senior Notes [Member] | 3.875% Unsecured Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Debt face amount												$ 250,000,000.0
Stated interest rate (percent)												3.875%
Debt discount												$ 200,000
Effective interest rate (percent)												3.885%
Debt issuance costs												$ 2,300,000
Senior Notes [Member] | Unsecured Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt redemption price			$ 333,200,000
Notes and bonds payable			300,000,000
Debt Redemption, accrued interest			6,400,000
Make-whole payment			26,800,000
Unaccreted discount and unamortized costs			1,200,000
Loss on early extinguishment of debt			$ (28,000,000)
Senior Notes [Member] | 5.0% Secured Note [Member]
Debt Instrument [Line Items]
Stated interest rate (percent)					5.00%
Amount of debt repaid					$ 10,200,000
Senior Notes [Member] | 5.45% Secured Note [Member]
Debt Instrument [Line Items]
Stated interest rate (percent)							5.45%
Amount of debt repaid							$ 15,000,000
Debt repayment, accrued interest							$ 100,000
Senior Notes [Member] | Medical Office Building [Member] | 5.0% Secured Note [Member]
Debt Instrument [Line Items]
Area of Real Estate Property | ft²					44,169
Senior Notes [Member] | Medical Office Building [Member] | 5.45% Secured Note [Member]
Debt Instrument [Line Items]
Area of Real Estate Property | ft²							73,548
Mortgages [Member] | 5.41% Mortgage Note Payable [Member]
Debt Instrument [Line Items]
Stated interest rate (percent)		5.25%		5.41%
Amount of debt repaid		$ 4,000,000		$ 16,300,000
Debt repayment, accrued interest				$ 100,000
Mortgages [Member] | Medical Office Building [Member] | 5.41% Mortgage Note Payable [Member]
Debt Instrument [Line Items]
Area of Real Estate Property | ft²		29,423		142,856