Notes Payable and Revolving Lines of Credit - Schedule of Information on Lines of Credit (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) extension	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Amount Drawn	$ 81,000,000	$ 94,000,000
Capacity	790,000,000
Credit Line 1
Debt Instrument [Line Items]
Amount Drawn	81,000,000
Capacity	$ 140,000,000
Interest Rate	4.00%
Number of extensions available | extension	1
Extension term	2 years
Credit Line 2
Debt Instrument [Line Items]
Amount Drawn	$ 0
Capacity	$ 650,000,000
Interest Rate	3.60%
Number of extensions available | extension	2
Extension term	6 months
LIBOR | Credit Line 1
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
LIBOR | Credit Line 2
Debt Instrument [Line Items]
Basis spread on variable rate	1.10%