Borrowings Under Credit Facilities and Commercial Paper Program - Unsecured Revolving Credit Facility (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Abstract]
Balance outstanding at year end	$ 325,000	$ 0	$ 1,588,600
Maximum amount outstanding at any month end	994,000	2,100,000	2,880,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 384,418	$ 497,014	$ 1,376,813
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	0.33%	2.09%	2.84%