Borrowings Under Credit Facilities and Commercial Paper Program - Aggregate Borrowings Under Unsecured Revolving Credit Facility and Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 291,000	$ 0	$ 291,000	$ 0
Maximum amount outstanding at any month end	990,000	0	990,000	2,100,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 470,935	$ 0	$ 255,114	$ 663,895
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	0.21%	0.00%	0.42%	2.09%