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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Balance outstanding at quarter end					$ 0	$ 0	$ 1,870,000	$ 1,870,000
Maximum amount outstanding at any month end	$ 685,000	$ 2,880,000	$ 2,100,000	$ 2,880,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 405,165	$ 1,807,631	$ 999,490	$ 1,301,883
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.57%	3.08%	2.08%	3.11%