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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 1,335,000	$ 1,312,000	$ 1,335,000	$ 1,312,000
Maximum amount outstanding at any month end	1,335,000	2,148,000	2,880,000	2,148,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 1,296,185	$ 1,519,000	$ 1,299,963	$ 819,516
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.82%	3.00%	3.02%	2.95%