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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 0	$ 0	$ 0	$ 0
Maximum amount outstanding at any month end	195,000	685,000	195,000	2,100,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 289,231	$ 405,165	$ 145,414	$ 999,490
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	0.76%	1.57%	0.76%	2.08%