Derivative Instruments - Interest rate derivatives and their fair values (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 700,000,000
Interest Rate Swap | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Value	(49,796,000)	$ (19,916,000)
Swap instrument one matures on December 17, 2021 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 25,000,000	25,000,000
Fixed Rate Per annum	1.989%
Fair Value	$ (447,000)	(209,000)
Swap instrument two matures on December 17, 2021 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	100,000,000
Fixed Rate Per annum	1.989%
Fair Value	$ (1,788,000)	(837,000)
Swap instrument three matures on December 17, 2021 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 75,000,000	75,000,000
Fixed Rate Per annum	1.989%
Fair Value	$ (1,342,000)	(627,000)
Swap instrument one matures on April 27, 2022 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 50,000,000	50,000,000
Fixed Rate Per annum	2.033%
Fair Value	$ (1,248,000)	(545,000)
Swap instrument two matures on April 27, 2022 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	100,000,000
Fixed Rate Per annum	2.029%
Fair Value	$ (2,490,000)	(1,081,000)
Swap instrument three matures on April 27, 2022 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 50,000,000	50,000,000
Fixed Rate Per annum	2.033%
Fair Value	$ (1,248,000)	(545,000)
Swap instrument one matures on December 17, 2023 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	100,000,000
Fixed Rate Per annum	2.617%
Fair Value	$ (7,191,000)	(4,007,000)
Swap instrument one matures on April 27, 2024 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	100,000,000
Fixed Rate Per annum	2.621%
Fair Value	$ (8,000,000)	(4,324,000)
Swap instrument one maturing October 18, 2026 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 70,000,000	0
Fixed Rate Per annum	0.968%
Fair Value	$ (2,174,000)	0
Swap instrument two maturing October18, 2026 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 30,000,000	0
Fixed Rate Per annum	0.973%
Fair Value	$ (938,000)	0
Swap instrument two matures on December 17, 2023 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 200,000,000	200,000,000
Fixed Rate Per annum	2.636%
Fair Value	$ (9,648,000)	(3,939,000)
Swap instrument two matures on April 27, 2024 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 200,000,000	200,000,000
Fixed Rate Per annum	2.642%
Fair Value	$ (9,500,000)	(3,802,000)
Swap instrument one maturing December 17, 2024 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 125,000,000	0
Fixed Rate Per annum	1.014%
Fair Value	$ (704,000)	0
Swap instrument two maturing December 17, 2024 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	0
Fixed Rate Per annum	1.035%
Fair Value	$ (584,000)	0
Swap instrument three maturing October 18, 2026 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 75,000,000	0
Fixed Rate Per annum	1.11%
Fair Value	$ (866,000)	0
Swap instrument one maturing April 27, 2025 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 100,000,000	0
Fixed Rate Per annum	1.088%
Fair Value	$ (540,000)	0
Swap instrument two maturing April 27, 2025 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 125,000,000	0
Fixed Rate Per annum	1.082%
Fair Value	$ (666,000)	0
Swap instrument four maturing October 18, 2026 | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 75,000,000	0
Fixed Rate Per annum	0.977%
Fair Value	$ (422,000)	$ 0