Debt, Preferred Equity and Other Investments (Preferred Equity Investments)(Details) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2015 USD ($) ft²	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) ft² extenstion_option segment	Dec. 31, 2014 USD ($) segment	Dec. 31, 2013 USD ($)
Preferred Equity Investments [Line Items]
Interest rate (as a percent)			10.23%
Future Funding Obligations			$ 5,580
Senior Financing			131,669
Carrying Value			42,176	$ 132,995
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year		$ 1,000	0	1,000	$ 7,000
Expensed			0	0	0
Recoveries			0	0	0
Charge-offs and reclassifications			0	(1,000)	(6,000)
Balance at end of period			$ 0	$ 0	$ 1,000
Number of portfolio segments of financial receivables (segment) | segment			1	1
Additional amount of financing receivables included in other assets			$ 168,300	$ 107,000
Approximate Square Feet (sqft) | ft²			12,576,446
West Coast Office Portfolio
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ownership interest in joint venture sold (as a percent)		43.74%
Valuation of interest in joint venture sold		$ 756,000
Gain (Loss) on Sale of Unconsolidated Joint Venture		85,600
131-137 Spring Street
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Approximate Square Feet (sqft) | ft²	68,342
Ownership percentage in disposed asset	80.00%
131-137 Spring Street
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ownership Interest (as a percent)			20.00%
Economic Interest (as a percent)			20.00%
Approximate Square Feet (sqft) | ft²			68,342
Acquisition price			$ 277,750
West Coast Office Portfolio Mortgage | Mortgages
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Floating rate debt		$ 526,300
Floating Rate Five Year Mortgage | Mortgages
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Floating rate debt	$ 141,000
Term (in Years)	5 years
Floating Rate Five Year Mortgage | Mortgages | LIBOR
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Interest rate (as a percent)	1.55%
Preferred Equity Investments
Preferred Equity Investments [Line Items]
Interest rate (as a percent)			7.86%
Preferred Equity, March 2018
Preferred Equity Investments [Line Items]
Future Funding Obligations			$ 0
Senior Financing			71,486
Carrying Value			9,967	9,954
Preferred Equity, November 2018
Preferred Equity Investments [Line Items]
Future Funding Obligations			5,580
Senior Financing			60,183
Carrying Value			32,209	0
Preferred Equity, Redeemed July 2015
Preferred Equity Investments [Line Items]
Future Funding Obligations			0
Senior Financing			0
Carrying Value			0	123,041
Mortgage Loan, February 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Carrying Value			0	0
Participating Financing due March 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Equity method investment			$ 100,200	$ 100,000
Number of one-year extension options | extenstion_option			2
Extension period (years)			1 year
Last extension period (years)			2 years