Debt - Additional Information (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Debt Narrative Detail Information
Letters of Credit Outstanding, Amount	$ 2.1
Amount Outsanding on Credit Facility	0.0
Receive Variable/Pay Fixed \| Net Realized Investments Gain (Loss)
Debt Narrative Detail Information
Loss on Derivative Contract Termination		$ 0.1
Long-term Debt
Debt Narrative Detail Information
Extinguishment of Debt, Amount		$ 3.4
Northwind Holdings LLC
Debt Narrative Detail Information
Principal Payments on Long-term Debt	$ 30.0
Notes Payable due 2018 \| Senior Notes
Debt Narrative Detail Information
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Long-term Debt, Current Maturities	$ 200.0
Junior Subordinated Debt \| Notes Payable Due 2058
Debt Narrative Detail Information
Proceeds from Issuance of Debt	$ 300.0
Debt Instrument, Interest Rate, Stated Percentage	6.25%

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- Definition

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

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- Definition

Amount of the required periodic payments applied to principal.

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- Definition

Gross amount of debt extinguished.

+ References

No definition available.

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

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

+ References

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- Definition

The loss recognized on termination of a contract.

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-Publisher FASB
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- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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