Notes Payable (New Credit Facility) (Details) - Credit Facility		3 Months Ended	12 Months Ended
	Jan. 03, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 500,000,000
Investment entity cap			$ 90,000,000
Minimum
Line of Credit Facility [Line Items]
Unencumbered interest coverage ratio			2.00
LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)			1.10%
LIBOR | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)			1.10%
LIBOR | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)			1.45%
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%
New Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,000,000,000
Additional borrowing capacity	$ 500,000,000
Remaining borrowing capacity		$ 997,000,000
Current borrowing capacity		1,000,000,000
Letters of credit outstanding		3,000,000
New Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit outstanding		$ 0
New Credit Facility | Minimum
Line of Credit Facility [Line Items]
Unencumbered interest coverage ratio	1.75
New Credit Facility | Maximum
Line of Credit Facility [Line Items]
Secured leverage ratio (percent)	40.00%
New Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.05%	1.05%
New Credit Facility | Federal Funds Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.50%
New Credit Facility | One-month LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.00%