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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Balance outstanding at quarter end					$ 0	$ 0	$ 1,335,000	$ 1,335,000
Maximum amount outstanding at any month end	$ 0	$ 1,335,000	$ 2,100,000	$ 2,880,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 0	$ 1,296,185	$ 663,895	$ 1,299,963
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	0.00%	2.82%	2.09%	3.02%