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Derivative Instruments - Interest rate swaps (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2021	Dec. 31, 2019
Derivative [Line Items]
Reclassification of other comprehensive income to interest expense	$ 3,400	$ (200)	$ 6,800	$ (1,200)
Forecast
Derivative [Line Items]
Reclassification of other comprehensive income to interest expense					$ 13,400
Interest Rate Swap
Derivative [Line Items]
Derivative instruments, notional amount	700,000		700,000
Interest Rate Swap \| Cash flow hedging
Derivative [Line Items]
Fair value	(55,502)		(55,502)			$ (19,916)
Interest Rate Swap \| Unsecured Credit Facility, Term Loan
Derivative [Line Items]
Aggregate principal amount	700,000		700,000
Interest Rate Swap \| Unsecured Credit Facility, Term Loan A, Matures on December 17, 2024
Derivative [Line Items]
Aggregate principal amount	225,000		225,000
Interest Rate Swap \| Unsecured Credit Facility, Term Loan B, Matures on April 27 2025
Derivative [Line Items]
Aggregate principal amount	225,000		225,000
Interest Rate Swap \| Unsecured Credit Facility, Term Loan C, Matures on October 18 2026
Derivative [Line Items]
Aggregate principal amount	$ 250,000		$ 250,000