Reinsurance - Narrative (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) reinsurer	Dec. 31, 2016 USD ($)
Ceded Credit Risk [Line Items]
Reinsurance recoverables	$ 3,424.5	$ 3,565.0
Number of reinsurers' insolvency for making company responsible for administering reinsurance business \| reinsurer	1
Reinsurance invested assets
Ceded Credit Risk [Line Items]
Invested assets held in trusts	$ 423.7	466.1
Largest Reinsurance Recoverable Balances
Ceded Credit Risk [Line Items]
Number of reinsurers with the largest reinsurance recoverable balances \| reinsurer	3
Reinsurance recoverables	$ 3,350.0
John Hancock
Ceded Credit Risk [Line Items]
Reinsurance recoverables	1,952.7	$ 1,908.2
Assets held in trust	$ 1,950.0

Represents the number of reinsurer insolvency for making company responsible for administering reinsurance business.

No definition available.

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- Definition

Represents the number of reinsurers with the largest reinsurance recoverable balances.

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 320
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75027294&loc=d3e22002-158495

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- Definition

Line items represent financial 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.

+ References

No definition available.

+ Details

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- Definition

Amount of the reinsurer's funds held on its behalf by the ceding insurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of the obligations thereunder.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.2,10)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),6)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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