Derivative Instruments - Interest rate derivatives and their fair values (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
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	(31,980,000)	$ (49,796,000)
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	$ (334,000)	(447,000)
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	$ (1,333,000)	(1,788,000)
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	$ (1,001,000)	(1,342,000)
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	$ (1,017,000)	(1,248,000)
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	$ (2,027,000)	(2,490,000)
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	$ (1,016,000)	(1,248,000)
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	$ (6,144,000)	(7,191,000)
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	$ (6,722,000)	(8,000,000)
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	70,000,000
Fixed rate per annum	0.968%
Fair value	$ 232,000	(2,174,000)
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	30,000,000
Fixed rate per annum	0.973%
Fair value	$ 100,000	(938,000)
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	$ (8,781,000)	(9,648,000)
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	$ (8,162,000)	(9,500,000)
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	125,000,000
Fixed rate per annum	1.014%
Fair value	$ 336,000	(704,000)
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	100,000,000
Fixed rate per annum	1.035%
Fair value	$ 244,000	(584,000)
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	75,000,000
Fixed rate per annum	1.11%
Fair value	$ 1,230,000	(866,000)
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	100,000,000
Fixed rate per annum	1.088%
Fair value	$ 399,000	(540,000)
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	125,000,000
Fixed rate per annum	1.082%
Fair value	$ 513,000	(666,000)
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	75,000,000
Fixed rate per annum	0.977%
Fair value	$ 1,503,000	$ (422,000)