Derivative Instruments (Narrative) (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($) contract	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Maximum length of time of cash flow hedge		75 months
Gain (loss) to be reclassified		$ 3,100,000
Interest rate swaps
Derivative [Line Items]
Number of terminated derivatives | contract	6
Gain on cash flow hedging instruments		100,000
Credit Risk Contract
Derivative [Line Items]
Assets needed for immediate settlement, aggregate fair value		300,000
Unsecured Debt | $300 Million Unsecured 2013 Term Loan
Derivative [Line Items]
Repayments of debt	$ 300,000,000	300,000,000
Proceeds from termination of interest rate swap agreement	800,000
Face amount of facility		300,000,000
Unsecured Debt | $300 Million Unsecured 2013 Term Loan | Interest rate swaps
Derivative [Line Items]
Net loss on interest rate cash flow hedge termination	$ 1,300,000
Unsecured Debt | Amended and Restated $300 Million Unsecured 2011 Term Loan
Derivative [Line Items]
Face amount of facility		300,000,000
Unsecured Debt | $250 Million Unsecured 2018 Term Loan
Derivative [Line Items]
Face amount of facility		250,000,000
Amount of hedged debt		$ 150,000,000