Long-term Debt and Short-term Borrowings (Narrative) (Details) - USD ($)					8 Months Ended
	Sep. 30, 2018	Dec. 31, 2015	Sep. 30, 2015	Apr. 28, 2015	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 729,000,000			$ 729,000,000	$ 800,600,000
FIve Year Senior Secured Credit Facility Maturing April 2020 [Member]
Debt Instrument [Line Items]
Credit agreement, term				5 years
Restated Credit Agreement
Debt Instrument [Line Items]
Required prepayment, percent of proceeds from asset sales and dispositions					100.00%
Required prepayment, proceeds from asset sales and disposition exceeding minimum amount					$ 10,000,000.0
Required prepayment, percent of proceeds from property insurance or condemnation awards					100.00%
Required prepayment, proceeds from property insurance or condemnation awards exceeding minimum amount					$ 10,000,000
Required prepayment, percent of proceeds from other non-permitted debt agreements					100.00%
Allowed Increase in Consolidated Leverage Ratio after a Material Acquisition					0.50
Permitted Increase in Size of Restated Credit Facility					$ 500,000,000
Restated Credit Agreement | Federal Funds
Debt Instrument [Line Items]
Basis spread on variable rate					0.50%
Restated Credit Agreement | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate, term					1 month
Basis spread on variable rate					1.00%
Restated Credit Agreement | Minimum
Debt Instrument [Line Items]
Base Amount of Dividends and/or Repurchase Shares in an Aggregate Amount in Dollars					$ 25,000,000
Base Amount of Dividends and/or Repurchase Shares in an Aggregate as a Percent of Consolidate Total Assets					1.00%
Consolidated Fixed Charge Coverage Ratio					1.25
Restated Credit Agreement | Maximum
Debt Instrument [Line Items]
Aggregate Amount of Investments Allowed as a Percentage of Consolidated Total Assets					15.00%
Aggregate Amount of Investments Allowed per the Restated Credit Agreement in Dollars					$ 500,000,000
U.S. Dollar Senior Secured Term Loan A, due April 2020 (floating interest rate of 1.88% at December 31, 2015) | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate		1.50%
Senior Secured Revolving Credit Facility, due April 2020
Debt Instrument [Line Items]
Letters of credit outstanding, amount		$ 8,900,000			8,900,000
Senior Secured Revolving Credit Facility, due April 2020 | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate		1.50%
Senior Secured Revolving Credit Facility, due April 2020 | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate		0.50%
Secured Debt [Member] | FIve Year Senior Secured Credit Facility Maturing April 2020 [Member]
Debt Instrument [Line Items]
Max borrowing capacity				$ 600,000,000
Secured Debt [Member] | U.S. Dollar Senior Secured Term Loan A, due April 2020 (floating interest rate of 1.88% at December 31, 2015)
Debt Instrument [Line Items]
Credit agreement, face amount				300,000,000
Debt, Long-term and Short-term, Combined Amount		$ 229,000,000			229,000,000	$ 0
Secured Debt [Member] | U.S. Dollar Senior Secured Term Loan A, due April 2020 (floating interest rate of 1.88% at December 31, 2015) | Minimum
Debt Instrument [Line Items]
Quarterly principal payment, based on annual percentage			5.00%
Secured Debt [Member] | U.S. Dollar Senior Secured Term Loan A, due April 2020 (floating interest rate of 1.88% at December 31, 2015) | Maximum | Forecast
Debt Instrument [Line Items]
Quarterly principal payment, based on annual percentage	12.50%
Secured Debt [Member] | Senior Secured Revolving Credit Facility, due April 2020
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 0			$ 0
Less than or equal to 2.50 to 1.00 | Restated Credit Agreement
Debt Instrument [Line Items]
Consolidated Leverage Ratio Required for any Additional Amount for Dividends and /or Repurchase Shares					2.50
Less than or equal to 3.50 to 1.00 and greater than 2.5 to 1.00 | Restated Credit Agreement
Debt Instrument [Line Items]
Additional Aggregate Amount not to Exceed in any Fiscal Year for Dividends and /or Repurchase Shares					$ 60,000,000
Revolving Credit Facility [Member] | Secured Debt [Member] | Senior Secured Revolving Credit Facility, due April 2020
Debt Instrument [Line Items]
Max borrowing capacity				300,000,000
Undrawn amounts, commitment fee (percent)		0.30%
Amount available for borrowings under the Restated Revolver Facility		$ 291,100,000			$ 291,100,000
Revolving Credit Facility [Member] | Secured Debt [Member] | Senior Secured Revolving Credit Facility, due April 2020 | Minimum
Debt Instrument [Line Items]
Undrawn amounts, commitment fee (percent)					0.25%
Revolving Credit Facility [Member] | Secured Debt [Member] | Senior Secured Revolving Credit Facility, due April 2020 | Maximum
Debt Instrument [Line Items]
Undrawn amounts, commitment fee (percent)					0.40%
Revolving Credit Facility [Member] | Secured Debt [Member] | USD Senior Secured Revolving Credit Facility, due May 2018
Debt Instrument [Line Items]
Max borrowing capacity				$ 250,000,000