Use of Derivative Financial Instruments - Risk Management and Unsecured Loan Swaps (Details) (USD $)	0 Months Ended
	Jan. 08, 2015 derivative	Mar. 31, 2015	Dec. 31, 2014
Unsecured Term Loan Swaps
Number of additional interest rate swaps	7
Notional Amount		$ 300,000,000
Fair value - assets		111,000	959,000
Fair value - liabilities		(4,037,000)	(873,000)
PNC Bank, National Association
Unsecured Term Loan Swaps
Notional amount assets		10,000,000
Fair value - assets		4,000
Fixed Interest Rate (as a percent)		0.7945%
Bank of America
Unsecured Term Loan Swaps
Notional amount assets		10,000,000
Fair value - assets		4,000
Fixed Interest Rate (as a percent)		0.7945%
UBS AG
Unsecured Term Loan Swaps
Notional amount assets		10,000,000
Fair value - assets		4,000
Fixed Interest Rate (as a percent)		0.7945%
Royal Bank of Canada
Unsecured Term Loan Swaps
Notional amount assets		10,000,000
Fair value - assets		4,000
Fixed Interest Rate (as a percent)		0.7945%
RJ Capital Services, Inc.
Unsecured Term Loan Swaps
Notional amount assets		10,000,000
Fair value - assets		3,000
Fixed Interest Rate (as a percent)		0.7975%
Bank of America
Unsecured Term Loan Swaps
Notional amount assets		25,000,000
Fair value - assets		35,000
Fixed Interest Rate (as a percent)		0.7525%
RJ Capital Services, Inc.
Unsecured Term Loan Swaps
Notional amount assets		25,000,000
Fair value - assets		51,000
Fixed Interest Rate (as a percent)		0.727%
Regions Bank
Unsecured Term Loan Swaps
Notional amount assets		25,000,000
Fair value - assets		6,000
Fixed Interest Rate (as a percent)		1.33%
Capital One, N.A.
Unsecured Term Loan Swaps
Notional amount liabilities		50,000,000
Fair value - liabilities		(801,000)
Fixed Interest Rate (as a percent)		1.703%
Capital One, N.A.
Unsecured Term Loan Swaps
Notional amount liabilities		25,000,000
Fair value - liabilities		(427,000)
Fixed Interest Rate (as a percent)		1.681%
Regions Bank
Unsecured Term Loan Swaps
Notional amount liabilities		25,000,000
Fair value - liabilities		(782,000)
Fixed Interest Rate (as a percent)		1.9925%
Royal Bank of Canada, effective March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities		25,000,000
Fair value - liabilities		(297,000)
Fixed Interest Rate (as a percent)		1.709%
Toronto-Dominion Bank, effective March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities		25,000,000
Fair value - liabilities		(297,000)
Fixed Interest Rate (as a percent)		1.7105%
Wells Fargo Bank, effective March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities		25,000,000
Fair value - liabilities		(340,000)
Fixed Interest Rate (as a percent)		1.828%
Toronto-Dominion Bank, effective September 10, 2017
Unsecured Term Loan Swaps
Notional amount liabilities		0
Fair value - liabilities		(678,000)
Fixed Interest Rate (as a percent)		2.2255%
Toronto-Dominion Bank, effective February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities		0
Fair value - liabilities		(59,000)
Fixed Interest Rate (as a percent)		2.4535%
Regions Bank, effective February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities		0
Fair value - liabilities		(175,000)
Fixed Interest Rate (as a percent)		2.475%
Capital One, N.A. effective February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities		0
Fair value - liabilities		$ (181,000)
Fixed Interest Rate (as a percent)		2.53%