Subsequent Events subsequent Events (Details) $ in Thousands				1 Months Ended	6 Months Ended
	Aug. 06, 2015 USD ($)	Jul. 31, 2015 USD ($) yr	Oct. 04, 2012 USD ($)	Aug. 07, 2015 USD ($) swaps yr	Jun. 30, 2015 USD ($) swaps yr	Jul. 29, 2015 USD ($) ft²
Interest Rate Swap [Member]
Subsequent Event [Line Items]
Notional Amount					$ 475,000
767 Fifth Partners LLC [Member]
Subsequent Event [Line Items]
Ownership Percentage by the Company				60.00%
767 Fifth Partners LLC [Member] | Interest Rate Swap [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps					5
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr					10
Average Fixed Interest Rate					2.793%
Notional Amount					$ 150,000
767 Fifth Partners LLC [Member] | Interest Rate Swap [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps				4
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr				10
Average Fixed Interest Rate				2.813%
Notional Amount				$ 100,000
Operating Partnership [Member] | Interest Rate Swap [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps					8
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr					10
Average Fixed Interest Rate					2.458%
Notional Amount					$ 325,000
Operating Partnership [Member] | Interest Rate Swap [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of interest rate swap agreements entered into (swap contracts) | swaps				1
Maximum period of hedging exposure to the variability in future cash flows for forecasted transactions (in years) | yr				10
Average Fixed Interest Rate				2.532%
Notional Amount				$ 25,000
200 Clarendon Street [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Non refundable option payment		$ 5,000
Ground Lease Term (in years) | yr		99
Previous ground lease term remaining (in years) | yr		45
Improvements to the Station that will be credited against ground lease obligation		$ 32,000
505 9th Street [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²						322,000
Ownership Percentage by the Company						50.00%
Sale Price Of Sold Property						$ 318,000
Mortgage loan assumed by buyer						$ 117,000
Fountain Square [Member]
Subsequent Event [Line Items]
Ownership Percentage by the Company			50.00%
Future Fixed Price to Acquire Remaining Portion of Joint venture			$ 102,000
Fountain Square [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Ownership Percentage by the Company	50.00%
Future Fixed Price to Acquire Remaining Portion of Joint venture	$ 100,900