Debt - Junior Subordinated Debt (Details) - USD ($)	1 Months Ended	12 Months Ended
Apr. 30, 2017	Dec. 31, 2017
7.875% Notes, due 2042
Debt Instrument [Line Items]
Face Value		$ 600,000,000
Interest rate		7.875%
Interest Rate Subsequent to Call Date		5.596%
8.125% Notes, due 2068
Debt Instrument [Line Items]
Face Value		$ 500,000,000
Interest rate		8.125%
Interest Rate Subsequent to Call Date		4.6025%
3 month Libor 2.125% Notes, due 2067
Debt Instrument [Line Items]
Face Value		$ 500,000,000
Interest Rate Subsequent to Call Date		2.125%
LIBOR \| 3 month Libor 2.125% Notes, due 2067
Debt Instrument [Line Items]
Variable interest rate	2.125%	2.125%
Interest rate	4.39%

Debt Instrument, Interest Rate, Stated Interest Rate After Call Date

No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ 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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Period Type:	instant

- 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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