Fair Value Measurements - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments not Recorded at Fair Value (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[1]	$ 745.2	$ 715.9
Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including securities due within one year	[1],[2]	$ 781.0	$ 783.2

[1] Carrying amount includes amounts which represent the unamortized debt issuance costs, discount or premium.

[2] Fair value is estimated using market prices, where available, and interest rates currently available for issuance of debt with similar terms and remaining maturities. The fair value measurements are classified as Level 2.