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
	Mar. 31, 2020	Mar. 31, 2019
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 845,000	$ 419,293
Maximum amount outstanding at any month end	2,100,000	1,150,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 1,593,816	$ 790,516
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.21%	3.22%