Subsequent Events (Details) - 2015 Credit Facility		1 Months Ended	12 Months Ended
	Feb. 27, 2018 USD ($)	Oct. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018	Mar. 31, 2018	Oct. 31, 2017 USD ($)
Subsequent Event [Line Items]
Debt instrument face amount		$ 650,000,000
Credit facility borrowing capacity						$ 350,000,000
Debt term		5 years
Line of credit facility expiration period			5 years
Credit facility maximum consolidated leverage ratio			3.75
Credit facility minimum consolidated fixed charge ratio			1.15
Subsequent Event
Subsequent Event [Line Items]
Debt instrument face amount	$ 650,000,000.0
Line of credit facility expiration period	5 years
Debt instrument, fee amount	$ 2,400,000
Credit facility maximum consolidated leverage ratio				3.50
Credit facility minimum consolidated fixed charge ratio					1.25
Line of Credit
Subsequent Event [Line Items]
Credit facility borrowing capacity			$ 300,000,000
Line of Credit | Subsequent Event
Subsequent Event [Line Items]
Credit facility borrowing capacity	275,000,000.0
Medium-term Notes | Subsequent Event
Subsequent Event [Line Items]
Debt instrument face amount	$ 308,400,000.0
Debt term	5 years