Debt (Details) (USD $)	1 Months Ended		1 Months Ended				9 Months Ended
	Sep. 30, 2014	Nov. 01, 2014 note	Sep. 30, 2014 3.375% Senior Notes due 2024 [Member]	Nov. 01, 2014 Unsecured Series A and B Senior Notes [Member]	Nov. 01, 2014 Series A Unsecured Senior Notes [Member]	Nov. 01, 2014 Series B Unsecured Senior Notes [Member]	Nov. 01, 2014 Unsecured Revolving Credit Facility [Member]	Nov. 01, 2014 Level 1 [Member]	Sep. 30, 2014 New York City Office [Member]	Sep. 22, 2014 New York City Office [Member]
Debt Instrument [Line Items]
Unamortized discount		$ 1,700,000
Unsecured senior notes interest rate			3.375%		6.38%	6.53%
Debt instrument			250,000,000	150,000,000	85,000,000	65,000,000
Debt redemption price, percentage			99.329%
Cash proceeds net of discount and other issuance fees and expenses	246,000,000
Sales and purchase agreement, property purchase price agreed upon									222,000,000	222,000,000
Number of unsecured senior notes held		2
Unsecured senior notes estimated fair value								426,000,000
Borrowing capacity for revolving credit facility							600,000,000
Revolving credit facility expiration date							Jun. 01, 2017
Sublimit for issuance of standby letters of credit							$ 300,000,000
Basis points margin over LIBOR							1.00%