Derivatives and Hedging Activities (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) derivative_instrument	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)
Derivative [Line Items]
Expected amount of derivative related interest to be reclassified to interest expense over the next 12 months	$ 2,928,000
Interest Rate Swap [Member]
Derivative [Line Items]
Settlement amount			$ 3,539,000
Reclassified from AOCI into Income			$ 1,837,000
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Derivative asset	4,000	$ 1,153,000
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Accounts Payable And Other Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	(1,943,000)	(749,000)
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Face Amount	$ 505,000,000
Number of derivative instruments held | derivative_instrument	10
Notional amount of derivatives	$ 505,000,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassified from AOCI into Income	5,466,000	5,525,000
Amount of loss recognized in OCI on derivatives (effective portion)	(7,809,000)	$ (9,560,000)
Unsecured Term Loan, Expiring February 2018 [Member] | Unsecured Term Loan [Member]
Derivative [Line Items]
Repayments of principal in year five	250,000,000
Repayments of principal after year five	$ 255,000,000