Debt and Credit Arrangements (Schedule of Long Term Debt) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total principal debt		$ 787.7	$ 797.3
Unamortized debt discount		(9.8)	(10.4)
Total		777.9	786.9
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 375.0	$ 375.0
Maturity date		Apr. 15, 2026	Apr. 15, 2026
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 62.6	$ 62.6
Maturity date		Oct. 15, 2025	Oct. 15, 2025
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 175.0	$ 175.0
Maturity date		Mar. 30, 2053	Mar. 30, 2053
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 50.1	$ 59.7
Maturity date	Feb. 03, 2027	Feb. 03, 2027	Feb. 03, 2027
FHLB borrowings (secured) [Member]
Debt Instrument [Line Items]
Total principal debt		$ 125.0	$ 125.0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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

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Amount, after accumulated amortization, of debt discount.

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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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