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
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Balance outstanding at quarter end			$ 0	$ 845,000
Maximum amount outstanding at any month end	$ 0	$ 2,100,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 0	$ 1,593,816
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	0.00%	2.21%