Derivative Instruments - Interest rate derivatives and their fair values (Details) - USD ($)	Sep. 30, 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	(55,502,000)	$ (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	$ (557,000)	(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	$ (2,227,000)	(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	$ (1,671,000)	(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	$ (1,470,000)	(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	$ (2,933,000)	(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	$ (1,470,000)	(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	$ (7,884,000)	(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	$ (8,727,000)	(4,324,000)
Swap instrument one matures on 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	$ (2,738,000)	0
Swap instrument two matures on October 18, 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	$ (1,180,000)	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	$ (9,812,000)	(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	$ (9,716,000)	(3,802,000)
Swap instrument one matures on 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	$ (850,000)	0
Swap instrument two matures on 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	$ (700,000)	0
Swap instrument three matures on 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	$ (1,256,000)	0
Swap instrument one matures on 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	$ (680,000)	0
Swap instrument two matures on 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	$ (842,000)	0
Swap instrument four matures on 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	$ (789,000)	$ 0