Derivative Instruments - Interest rate derivatives and their fair values (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount of derivative	$ 700,000
Interest Rate Swap | Cash flow hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value	(58,290)	$ (19,916)
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	25,000
Fixed Rate Per annum	1.989%
Expiration Date	Dec. 17, 2021
Fair value	$ (672)	(209)
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	100,000
Fixed Rate Per annum	1.989%
Expiration Date	Dec. 17, 2021
Fair value	$ (2,689)	(837)
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	75,000
Fixed Rate Per annum	1.989%
Expiration Date	Dec. 17, 2021
Fair value	$ (2,017)	(627)
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	50,000
Fixed Rate Per annum	2.033%
Expiration Date	Apr. 27, 2022
Fair value	$ (1,703)	(545)
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	100,000
Fixed Rate Per annum	2.029%
Expiration Date	Apr. 27, 2022
Fair value	$ (3,400)	(1,081)
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	50,000
Fixed Rate Per annum	2.033%
Expiration Date	Apr. 27, 2022
Fair value	$ (1,704)	(545)
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	100,000
Fixed Rate Per annum	2.617%
Expiration Date	Dec. 17, 2023
Fair value	$ (8,502)	(4,007)
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	100,000
Fixed Rate Per annum	2.621%
Expiration Date	Apr. 27, 2024
Fair value	$ (9,331)	(4,324)
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
Fixed Rate Per annum	0.968%
Expiration Date	Oct. 18, 2026
Fair value	$ (2,818)
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
Fixed Rate Per annum	0.973%
Expiration Date	Oct. 18, 2026
Fair value	$ (1,215)
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	200,000
Fixed Rate Per annum	2.636%
Expiration Date	Dec. 17, 2023
Fair value	$ (9,783)	(3,939)
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	200,000
Fixed Rate Per annum	2.642%
Expiration Date	Apr. 27, 2024
Fair value	$ (9,653)	$ (3,802)
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
Fixed Rate Per annum	1.014%
Expiration Date	Dec. 17, 2024
Fair value	$ (799)
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
Fixed Rate Per annum	1.035%
Expiration Date	Dec. 17, 2024
Fair value	$ (658)
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
Fixed Rate Per annum	1.11%
Expiration Date	Oct. 18, 2026
Fair value	$ (1,187)
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
Fixed Rate Per annum	1.088%
Expiration Date	Apr. 27, 2025
Fair value	$ (639)
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
Fixed Rate Per annum	1.082%
Expiration Date	Apr. 27, 2025
Fair value	$ (790)
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
Fixed Rate Per annum	0.977%
Expiration Date	Oct. 18, 2026
Fair value	$ (730)