Derivative Financial Instruments - Risk Management and Unsecured Loan Swaps (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Interest Rate Swaps
Unsecured Term Loan Swaps
Notional amount assets	$ 0	$ 250,000
Notional amount liabilities	1,125,000	850,000
Notional amount liabilities	1,125,000	$ 850,000
Interest Rate Derivative, Royal Bank of Canada, March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities	25,000
Derivative Liability, Fair Value, Gross Liability	$ (182)
Fixed Interest Rate (as a percent)	1.709%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (182)
Fixed Interest Rate (as a percent)	1.709%
Interest Rate Derivative, The Toronto-Dominion Bank, March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (182)
Fixed Interest Rate (as a percent)	1.7105%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (182)
Fixed Interest Rate (as a percent)	1.7105%
Interest Rate Derivative, The Toronto-Dominion Bank, September 10, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 100,000
Derivative Liability, Fair Value, Gross Liability	$ (970)
Fixed Interest Rate (as a percent)	2.2255%
Notional amount liabilities	$ 100,000
Derivative Liability, Fair Value, Gross Liability	$ (970)
Fixed Interest Rate (as a percent)	2.2255%
Interest Rate Derivative, Wells Fargo Bank, March 20, 2015
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (639)
Fixed Interest Rate (as a percent)	1.828%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (639)
Fixed Interest Rate (as a percent)	1.828%
Interest Rate Derivative, The Toronto-Dominion Bank, February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (874)
Fixed Interest Rate (as a percent)	2.4535%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (874)
Fixed Interest Rate (as a percent)	2.4535%
Interest Rate Derivative, Regions Bank, February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,766)
Fixed Interest Rate (as a percent)	2.475%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,766)
Fixed Interest Rate (as a percent)	2.475%
Interest Rate Derivative, Capital One, February 14, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,808)
Fixed Interest Rate (as a percent)	2.53%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,808)
Fixed Interest Rate (as a percent)	2.53%
Interest Rate Derivative, The Toronto-Dominion Bank, October 30, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (979)
Fixed Interest Rate (as a percent)	1.8485%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (979)
Fixed Interest Rate (as a percent)	1.8485%
Interest Rate Derivative, Royal Bank of Canada, October 30, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (980)
Fixed Interest Rate (as a percent)	1.8505%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (980)
Fixed Interest Rate (as a percent)	1.8505%
Interest Rate Derivative, Wells Fargo Bank, October 30, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (980)
Fixed Interest Rate (as a percent)	1.8505%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (980)
Fixed Interest Rate (as a percent)	1.8505%
Interest Rate Derivative, PNC Bank, $25m, October 30, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (978)
Fixed Interest Rate (as a percent)	1.8485%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (978)
Fixed Interest Rate (as a percent)	1.8485%
Interest Rate Derivative, PNC Bank, $50m, October 30, 2017
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,956)
Fixed Interest Rate (as a percent)	1.8475%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (1,956)
Fixed Interest Rate (as a percent)	1.8475%
Interest Rate Derivative, TD Bank, $75m, Sep 29, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (272)
Fixed Interest Rate (as a percent)	0.275%
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (272)
Fixed Interest Rate (as a percent)	0.275%
Interest Rate Derivative, Wells Fargo Bank, $75m, Sep 29, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (279)
Fixed Interest Rate (as a percent)	0.279%
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (279)
Fixed Interest Rate (as a percent)	0.279%
Interest Rate Derivative, TD Bank, $75m, March 19, 2021
Unsecured Term Loan Swaps
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (230)
Fixed Interest Rate (as a percent)	0.275%
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (230)
Fixed Interest Rate (as a percent)	0.275%
Interest Rate Derivative, Wells Fargo Bank, $75m, March 19, 2021
Unsecured Term Loan Swaps
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (239)
Fixed Interest Rate (as a percent)	0.28%
Notional amount liabilities	$ 75,000
Derivative Liability, Fair Value, Gross Liability	$ (239)
Fixed Interest Rate (as a percent)	0.28%
Interest Rate Derivative, The Toronto-Dominion Bank, July 26, 2019 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.918%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.918%
Interest Rate Derivative, PNC Bank, July 26, 2019 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.919%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.919%
Interest Rate Derivative, Bank of Montreal, July 26, 2019 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.919%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,568)
Fixed Interest Rate (as a percent)	2.919%
Interest Rate Derivative, US Bank, July 26, 2019 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (2,284)
Fixed Interest Rate (as a percent)	2.919%
Notional amount liabilities	$ 25,000
Derivative Liability, Fair Value, Gross Liability	$ (2,284)
Fixed Interest Rate (as a percent)	2.919%
Interest Rate Derivative, Wells Fargo Bank, July 15, 2020 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,412)
Fixed Interest Rate (as a percent)	2.246%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,412)
Fixed Interest Rate (as a percent)	2.246%
Interest Rate Derivative, US Bank, July 15, 2020 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,411)
Fixed Interest Rate (as a percent)	2.2459%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,411)
Fixed Interest Rate (as a percent)	2.2459%
Interest Rate Derivative, Regions Bank, July 15, 2020 [Member]
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,411)
Fixed Interest Rate (as a percent)	2.2459%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (4,411)
Fixed Interest Rate (as a percent)	2.2459%
Interest Rate Derivative, Bank of Montreal, July 15, 2020
Unsecured Term Loan Swaps
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (3,276)
Fixed Interest Rate (as a percent)	1.7165%
Notional amount liabilities	$ 50,000
Derivative Liability, Fair Value, Gross Liability	$ (3,276)
Fixed Interest Rate (as a percent)	1.7165%