Debt (Schedule of Long Term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total principal debt	$ 814.3	$ 905.4
Unamortized debt discount	(1.5)	(1.9)
Total	812.8	903.5
Federal Home Loan Bank of Boston [Member]
Debt Instrument [Line Items]
Total principal debt	125.0	125.0
Senior debentures maturing June 15, 2021 [Member]
Debt Instrument [Line Items]
Total principal debt	300.0	300.0
Senior debentures maturing March 1, 2020 [Member]
Debt Instrument [Line Items]
Total principal debt	80.0	164.6
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt	74.6	81.1
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt	175.0	175.0
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt	$ 59.7	$ 59.7

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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