Derivative Instruments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Maximum period of extension term			78 months
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Gross derivative assets	$ 4,069,000		$ 4,069,000		$ 688,000
Gross derivative liabilities	0		0		(1,478,000)
Net derivative asset/(liability)	4,069,000		4,069,000		$ (790,000)
Amount of gain recognized in OCI	1,145,000	$ 175,000	4,408,000	$ 307,000
Total amount of Interest Expense presented in the consolidated statements of income	15,849,000	16,183,000	45,294,000	52,661,000
Total amount of Loss on Extinguishment of Debt presented in the consolidated statements of income	0	0	1,680,000	0
Loss on write off of discount and unamortized debt issuance costs			400,000
Gain (loss) to be reclassified	3,600,000		3,600,000
Gain (loss) from hedge ineffectiveness	0	0	0	0
Interest rate swaps
Derivative [Line Items]
Total Notional Amount (in millions)	450,000,000		450,000,000
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Total amount of Interest Expense presented in the consolidated statements of income	15,849,000	16,183,000	45,294,000	52,661,000
Interest rate swaps | Interest Expense
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Amount of gain recognized in OCI	1,145,000	175,000	4,408,000	307,000
Amount of gain/(loss) reclassified from OCI	434,000	(653,000)	885,000	(2,936,000)
Interest rate swaps | Gain (Loss) On Extinguishment Of Debt
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Amount of gain/(loss) reclassified from OCI	$ 0	$ 0	$ (1,258,000)	$ 0
$250 Million Unsecured 2018 Term Loan | Interest rate swaps
Derivative [Line Items]
Number of Swap Agreements | contract	3		3
Total Notional Amount (in millions)	$ 150,000,000		$ 150,000,000
Unsecured Debt | Amended and Restated $300 Million Unsecured 2011 Term Loan
Derivative [Line Items]
Face amount of debt instrument	$ 300,000,000		$ 300,000,000
Unsecured Debt | Amended and Restated $300 Million Unsecured 2011 Term Loan | Interest rate swaps
Derivative [Line Items]
Number of Swap Agreements | contract	3		3
Total Notional Amount (in millions)	$ 300,000,000		$ 300,000,000
Unsecured Debt | $250 Million Unsecured 2018 Term Loan
Derivative [Line Items]
Face amount of debt instrument	250,000,000		250,000,000
Hedged portion of long-term debt	150,000,000		150,000,000
Total Notional Amount (in millions)	$ 150,000,000		$ 150,000,000
Unsecured Debt | $250 Million Unsecured 2018 Term Loan | Interest rate swaps
Derivative [Line Items]
Number of Swap Agreements | contract	2		2
Total Notional Amount (in millions)	$ 100,000,000		$ 100,000,000
Unsecured Debt | $250 Million Unsecured 2018 Term Loan Maturing 2020 | Interest rate swaps
Derivative [Line Items]
Number of Swap Agreements | contract	1		1
Total Notional Amount (in millions)	$ 50,000,000		$ 50,000,000