Financial Instruments and Fair Value Measurements - Summary or Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying value
Fair value of financial assets and liabilities
Mortgages payable assumed in connection with acquisitions	$ 295,900	$ 298,400
Notes and bonds payable	6,299,900	5,400,000
Estimated fair value
Fair value of financial assets and liabilities
Mortgages payable assumed in connection with acquisitions	305,300	305,700
Notes and bonds payable	6,713,700	5,430,000
Mortgages Payable
Fair value of financial assets and liabilities
Unamortized balance of non-cash net premiums	3,700	4,400
Remaining balance of deferred financing costs at period end	156	183
Notes and bonds payable
Fair value of financial assets and liabilities
Unamortized balance of non-cash net premiums	6,800	10,500
Remaining balance of deferred financing costs at period end	$ 38,600	$ 33,700