Notes Payable (Credit Facility) (Details) - Credit Facility	6 Months Ended
	Jun. 30, 2019 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,000,000,000
Minimum fixed charge coverage ratio	1.50
Available borrowing capacity	$ 936,000,000.0
LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.05%
Federal Funds Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.50%
One-month LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.00%
Minimum
Line of Credit Facility [Line Items]
Minimum unencumbered interest coverage ratio	1.75
Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.05%
Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.10%
Maximum
Line of Credit Facility [Line Items]
Secured leverage ratio (percent)	40.00%
Leverage ratio (percent)	60.00%
Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.45%
Maximum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.45%