Fair Value Disclosures - Summary of the Change in Balance Sheet Carrying Value Associated with Level 3 Financial Assets Carried at Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	$ 18.2	$ 28.2
Total (losses) gains (realized/unrealized) included in earnings	0.1	1.5
Net unrealized gains (losses) included in other comprehensive income	0.1	(1.6)
Purchases	29.0	5.5
Sales	(8.6)	(13.9)
Transfers in	0.3	0.1
Transfers out	(20.3)	(1.6)
Balance, end of period	18.8	18.2
Other assets
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	0.3	0.4
Total (losses) gains (realized/unrealized) included in earnings	(0.1)	(0.1)
Net unrealized gains (losses) included in other comprehensive income	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	0.2	0.3
States, municipalities and political subdivisions
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	0.0	0.0
Total (losses) gains (realized/unrealized) included in earnings	0.1	0.0
Net unrealized gains (losses) included in other comprehensive income	(0.1)	0.0
Purchases	29.0	1.3
Sales	(7.4)	(1.3)
Transfers in	0.0	0.0
Transfers out	(16.9)	0.0
Balance, end of period	4.7	0.0
Commercial mortgage-backed
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	3.2	0.1
Total (losses) gains (realized/unrealized) included in earnings	0.1	0.0
Net unrealized gains (losses) included in other comprehensive income	0.0	(0.2)
Purchases	0.0	3.4
Sales	0.0	(0.1)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	3.3	3.2
U.S. corporate
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	3.6	10.7
Total (losses) gains (realized/unrealized) included in earnings	0.0	0.5
Net unrealized gains (losses) included in other comprehensive income	0.3	(0.6)
Purchases	0.0	0.8
Sales	(0.6)	(6.3)
Transfers in	0.3	0.1
Transfers out	(3.4)	(1.6)
Balance, end of period	0.2	3.6
Foreign corporate
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	10.0	15.9
Total (losses) gains (realized/unrealized) included in earnings	0.0	1.1
Net unrealized gains (losses) included in other comprehensive income	(0.1)	(0.8)
Purchases	0.0	0.0
Sales	(0.6)	(6.2)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	9.3	10.0
Non-redeemable preferred stocks
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	1.1	1.1
Total (losses) gains (realized/unrealized) included in earnings	0.0	0.0
Net unrealized gains (losses) included in other comprehensive income	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	$ 1.1	$ 1.1

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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-Name Accounting Standards Codification
-Topic 820
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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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-Name Accounting Standards Codification
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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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