Borrowings Under Credit Facilities and Commercial Paper Program (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Abstract]
Balance outstanding at year end	$ 0	$ 1,588,600	$ 1,147,000
Maximum amount outstanding at any month end	2,100,000	2,880,000	2,148,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 497,014	$ 1,376,813	$ 950,581
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.09%	2.84%	3.07%