Acquisitions Acquisitions - Additional Information (Details) (USD $)	0 Months Ended	9 Months Ended
	Jul. 15, 2014	Sep. 30, 2014	Sep. 30, 2013
Business Acquisition [Line Items]
Total consideration paid	$ 733,600,000
Debt assumed	167,684,000	182,851,000	0
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable		379,328,000	0
112 West 34th Street [Member]
Business Acquisition [Line Items]
Total consideration paid	423,600,000
Debt assumed	87,700,000
Payments to Acquire Businesses, Gross	106,900,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	229,000,000
112 West 34th Street [Member] | Common Class A [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,217,685
112 West 34th Street [Member] | Common Class B [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	77,945
112 West 34th Street [Member] | Series ES Operating Partnership Units [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	12,457,379
1400 Broadway [Member]
Business Acquisition [Line Items]
Total consideration paid	310,000,000
Debt assumed	80,000,000
Payments to Acquire Businesses, Gross	79,700,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	$ 150,300,000
1400 Broadway [Member] | Common Class A [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,338,488
1400 Broadway [Member] | Common Class B [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	32,452
1400 Broadway [Member] | Series ES Operating Partnership Units [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	7,658,516