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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 1,870,000	$ 540,000	$ 1,870,000	$ 540,000
Maximum amount outstanding at any month end	2,880,000	685,000	2,880,000	865,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 1,807,631	$ 562,747	$ 1,301,883	$ 463,978
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	3.08%	3.04%	3.11%	2.91%