Acquisitions Schedule of allocation of the purchase price for assets and liabilities acquired (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended
	Jul. 15, 2014	Oct. 06, 2013	Dec. 31, 2014	Dec. 31, 2013
Net assets acquired:
Building and improvements	$ 354,429
Acquired below-market ground leases	334,178
Acquired above-market leases	13,088
Acquired in place lease value and deferred leasing costs	88,374
Goodwill			491,479	491,479
Mortgage notes payable	(182,851)
Acquired below-market leases	(36,553)
Other liabilities, net of other assets	(4,749)
Total Acquisitions [Member]
Consideration paid:
Cash and issuance of Class A Common Stock, Class B Common Stock, and OP units	565,916	1,047,487
Debt assumed	167,684	124,354
Total consideration paid	733,600	1,171,841
Net assets acquired:
Land		91,435
Building and improvements		516,344
Acquired below-market ground leases		62,738
Acquired above-market leases		72,123
Acquired in place lease value and deferred leasing costs		186,415
Goodwill		491,479
Other assets, net of other liabilities		6,061
Mortgage notes payable		(136,226)
Acquired below-market leases		(134,651)
Total net assets acquired	565,916	1,155,718
Business Combination, Bargain Purchase [Abstract]
Total		108,231
Leasehold Positions [Member]
Business Combination, Bargain Purchase [Abstract]
Total		35,147
1333 Broadway Associates [Member]
Business Combination, Bargain Purchase [Abstract]
Total		40,962
1350 Broadway Associates [Member]
Business Combination, Bargain Purchase [Abstract]
Total		$ 32,122