General - Long-Term Debt, Finance Lease and Deferred Financing Obligations - Senior Notes (Details) - Senior Notes - 4.400% Senior Notes due 2024 - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 22, 2017
Debt
Debt instrument, face amount				$ 400.0
Stated interest rate (as a percent)				4.40%
Debt instrument, repurchased face amount	$ 28.8	$ 28.8	$ 11.1
Gain (loss) on repurchase of debt instrument		0.7	0.3
Long-term debt	$ 359.6	$ 359.6	$ 388.4
Interest rate during the period (as a percent)	4.90%	4.90%
Debt instrument, maturity date		Sep. 22, 2024
Redemption price of the principal amount redeemed (as a percent)		100.00%

Face (par) amount of debt instrument at time of issuance.

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The average effective interest rate during the reporting period.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

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Face (par) amount of the original debt instrument that was repurchased.

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Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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