Notes Payable (Credit Facility) (Details) - Credit Facility			9 Months Ended	12 Months Ended
	Jan. 03, 2018 USD ($)	Jan. 02, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 500,000,000
Minimum shareholders' equity		$ 1,000,000,000
Minimum
Line of Credit Facility [Line Items]
Unencumbered interest coverage ratio		2.0		2.0
Fixed charge coverage ratio		1.50
Annual facility fee rate (percent)		0.15%
Maximum
Line of Credit Facility [Line Items]
Leverage ratio (percent)		60.00%
Annual facility fee rate (percent)		0.30%
LIBOR | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.05%	1.10%
LIBOR | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.45%	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%
Base Rate | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.10%	0.10%
Base Rate | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.45%	0.45%
New Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,000,000,000
Additional borrowing capacity	$ 500,000,000
Line of credit outstanding			$ 0
Remaining borrowing capacity			998,000,000
Current borrowing capacity			1,000,000,000
Letters of credit outstanding			$ 2,000,000
New Credit Facility | Minimum
Line of Credit Facility [Line Items]
Unencumbered interest coverage ratio	1.75
New Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)			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%