Debt Summary of Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 836.9	$ 1,189.5
Deferred issuance costs	(6.5)	(10.8)
Total long-term debt	830.4	1,178.7
Interest payments made	57.8	49.5
Federal Home Loan Bank Advances (FHLB)
Debt Instrument [Line Items]
Long-term debt, gross	155.0	155.0
Interest payments made	3.0	2.4
5% Convertible Senior Notes due 2017
Debt Instrument [Line Items]
Long-term debt, gross	$ 0.0	145.0
Stated interest rate (in hundredths)	5.00%
Interest payments made	$ 3.6	10.6
2% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 0.0	207.6
Stated interest rate (in hundredths)	2.00%
Interest payments made	$ 2.1	9.1
Senior Notes \| 5.75% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, gross	$ 425.0	425.0
Stated interest rate (in hundredths)	5.75%
Interest payments made	$ 25.1	0.0
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063
Debt Instrument [Line Items]
Long-term debt, gross	$ 256.9	256.9
Stated interest rate (in hundredths)	9.00%
Interest payments made	$ 23.1	27.4
Line of Credit
Debt Instrument [Line Items]
Interest payments made	$ 0.9	$ 0.0

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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