Debt Notes and Debentures (Details) $ in Millions	Mar. 31, 2018 USD ($)
SEC Debt Shelf Registration [Member]
Debt Instrument [Line Items]
Remaining Debt Securities Authorized by the Board	$ 1,000
Junior Subordinated Debt
Debt Instrument [Line Items]
Subordinated notes related to financial covenants	500
Four Point Zero Five Percent Debentures Due June 15, 2048 [Member] [Member] [Member] \| Corporate Debt Securities [Member]
Debt Instrument [Line Items]
Debentures issued	$ 750
Interest rate on issued debentures	4.05%

Certain issued and outstanding junior subordinated notes pertaining to certain financial covenants.

No definition available.

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- Definition

Remaining Debt Securities Authorized by the Board

+ References

No definition available.

+ Details

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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- Definition

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

+ 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)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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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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- Details

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- Details

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- Details

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- Details

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