Unsecured Line of Credit Payable - Borrowings and Repayments (Details)	3 Months Ended
	Mar. 31, 2019 USD ($)
Line of Credit Facility, Increase (Decrease) During the Period [Roll Forward]
Balance at December 31, 2018	$ 188,000,000
Balance at March 31, 2019	228,000,000
Credit facility 2018
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	1,500,000,000.0
Line of Credit | Credit facility 2018
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	700,000,000.0
Line of Credit Facility, Increase (Decrease) During the Period [Roll Forward]
Balance at December 31, 2018	188,000,000
Borrowings	71,000,000
Repayments	(31,000,000)
Balance at March 31, 2019	$ 228,000,000