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Investments - Available-for-sale Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis	$ 1,193.2	$ 529.2

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- Definition

Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration