Investments - Summary of Loan-To-Value and Average Debt-Service Coverage Ratios (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Commercial mortgage loans
Net commercial mortgage loans on real estate	$ 78.7	$ 98.4
70% and less \| Maximum
Commercial mortgage loans
Loan-to-value percentage	70.00%	70.00%
81 - 95% \| Minimum
Commercial mortgage loans
Loan-to-value percentage	81.00%	81.00%
81 - 95% \| Maximum
Commercial mortgage loans
Loan-to-value percentage	95.00%	95.00%
Greater than 95% \| Minimum
Commercial mortgage loans
Loan-to-value percentage	95.00%	95.00%
Commercial mortgage loans
Commercial mortgage loans
Gross commercial mortgage loans on real estate		$ 100.1
Less valuation allowance	$ (0.4)	(1.7)
Net commercial mortgage loans on real estate	78.7	$ 98.4
% of Gross Mortgage Loans		100.00%
Debt- Service Coverage Ratio		1.88
Decrease in loan valuation allowance	1.3
Commercial mortgage loans \| Minimum
Commercial mortgage loans
Gross commercial mortgage loans on real estate	0.1	$ 0.1
Commercial mortgage loans \| Maximum
Commercial mortgage loans
Gross commercial mortgage loans on real estate	8.4	6.6
Commercial mortgage loans \| 70% and less
Commercial mortgage loans
Gross commercial mortgage loans on real estate	$ 79.1	$ 92.3
% of Gross Mortgage Loans	100.00%	92.20%
Debt- Service Coverage Ratio	2.11	1.80
Commercial mortgage loans \| 81 - 95%
Commercial mortgage loans
Gross commercial mortgage loans on real estate		$ 3.0
% of Gross Mortgage Loans		3.00%
Debt- Service Coverage Ratio		1.11
Commercial mortgage loans \| Greater than 95%
Commercial mortgage loans
Gross commercial mortgage loans on real estate		$ 4.8
% of Gross Mortgage Loans		4.80%
Debt- Service Coverage Ratio		3.86
Commercial mortgage loans \| Investment portfolio \| Geographic concentration risk \| California, Alabama and Utah
Commercial mortgage loans
Concentration percentage	41.00%
Fixed maturity securities
Commercial mortgage loans
Fixed maturity securities on deposit with various governmental authorities	$ 6.2	$ 6.1

Represents percentage of classified loans to total gross commercial mortgage loans.

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Represents the ratio of cash available for debt servicing of interest, principal and lease payments for commercial mortgage loans.

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Represents the ratio of a loan underwritten to a value of an asset purchased.

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Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The increase (decrease) in the allowance for credit losses related to financing receivables.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance of mortgage loans on existing land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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