Notes Payable (Credit Facility) (Details) - Credit Facility	12 Months Ended
	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000,000
Minimum shareholders' equity	$ 1,000,000,000.0
LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.10%
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]
Unencumbered interest coverage ratio	2.00
Fixed charge coverage ratio	1.50
Annual facility fee (percent)	0.15%
Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.10%
Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.10%
Maximum
Line of Credit Facility [Line Items]
Leverage ratio (percent)	60.00%
Annual facility fee (percent)	0.30%
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%