Investments - Held-to-maturity Security - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2015
Schedule Of Held To Maturity Securities [Line Items]
Debt instrument maturity date		Jul. 15, 2022
Debt instrument, interest rate, stated percentage		4.75%
Held-to-maturity debt security estimated unrealized holding gain	$ 42.3
LLC Note [Member]
Schedule Of Held To Maturity Securities [Line Items]
Debt instrument maturity date	Dec. 31, 2030
Debt instrument, interest rate, stated percentage	4.50%
Surplus Note [Member]
Schedule Of Held To Maturity Securities [Line Items]
Debt instrument maturity date	Dec. 31, 2030
Debt instrument, interest rate, stated percentage	4.50%

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of unrecognized gain before loss on investments in held-to-maturity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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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.

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