Derivative Instruments (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Net gain/ (loss) related to hedge ineffectiveness and terminations of cash flow hedges	$ 0	$ 37,000	$ 0
Maximum length of time of cash flow hedge	36 months
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Gross derivative assets	$ 0	0
Interest rate swaps	8,169,000	9,993,000
Net derivative liability	8,169,000	9,993,000
Amount that will be reclassified from accumulated other comprehensive income over the next twelve months	4,100,000
Unsecured Debt [Member]
Derivative [Line Items]
Total notional amount of forward starting interest rate agreements	600,000,000
Interest Rate Contract [Member] | Interest Expense [Member]
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Amount of gain/(loss) recognized in OCI on derivatives	(4,126,000)	(12,509,000)	(17,122,000)
Amount of previously recorded loss reclassified from accumulated OCI into interest expense	4,548,000	$ 5,875,000	$ 5,145,000
Forward Starting Interest Rate Swap [Member]
Derivative [Line Items]
Total notional amount of forward starting interest rate agreements	$ 250,000,000
Number of forward starting interest rate swaps settled | contract	4
Forward Starting Interest Rate Swap [Member] | Interest Expense [Member]
Derivative [Line Items]
Settlement value of forward starting interest rate swaps	$ 100,000
Credit Risk Contract [Member]
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Assets needed for immediate settlement, aggregate fair value	8,400,000
$300 Million Unsecured 2011 Term Loan [Member] | Unsecured Debt [Member]
Derivative [Line Items]
Face amount of facility	300,000,000
$300 Million Unsecured 2011 Term Loan [Member] | Interest Rate Swap 7 through 9 [Member] | Unsecured Debt [Member]
Derivative [Line Items]
Total notional amount of forward starting interest rate agreements	$ 300,000,000
Number of Swap Agreements | contract	3
$300 Million Unsecured 2013 Term Loan [Member] | Unsecured Debt [Member]
Derivative [Line Items]
Face amount of facility	$ 300,000,000
$300 Million Unsecured 2013 Term Loan [Member] | Interest Rate Swap 1 through 4 [Member] | Unsecured Debt [Member]
Derivative [Line Items]
Total notional amount of forward starting interest rate agreements	$ 200,000,000
Number of Swap Agreements | contract	4
$300 Million Unsecured 2013 Term Loan [Member] | Interest Rate Swap 5 through 6 [Member] | Unsecured Debt [Member]
Derivative [Line Items]
Total notional amount of forward starting interest rate agreements	$ 100,000,000
Number of Swap Agreements | contract	2
$160.0 Million Fixed Rate Loan [Member] | Secured Debt [Member]
Derivative [Line Items]
Face amount of facility	$ 160,000,000
Total notional amount of forward starting interest rate agreements	250,000,000
Net loss from settlement of forward starting interest rate swaps	1,300,000
Net gain/ (loss) related to hedge ineffectiveness and terminations of cash flow hedges	$ 100,000
Term of loan	7 years