Debt (details) - Junior Sub Debt $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Debt Instrument
Unamortized fair value adjustment, debt	$ 47	$ 49
Junior subordinated debt [Member]
Debt Instrument
Number of junior subordinated debentures that are not fixed-to-floating rate debentures \| item	3
Number of separate trusts that issued preferred securities and used proceeds to purchase the Company's subordinated debentures that are not fixed-to-floating rate \| item	3
Impact of amortization of the fair value adjustment on interest expense	$ 2	$ 1
7.625% Junior subordinated debentures due December 15, 2027 [Member]
Debt Instrument
Interest rate (percent)	7.625%	7.625%
Debt, effective interest rate (percent)	6.147%	6.147%
Unamortized fair value adjustment, debt	$ 14	$ 15
8.50% Junior subordinated debentures due December 15, 2045 [Member]
Debt Instrument
Interest rate (percent)	8.50%	8.50%
Debt, effective interest rate (percent)	6.362%	6.362%
Unamortized fair value adjustment, debt	$ 15	$ 15
8.312% Junior subordinated debentures due July 1, 2046 [Member]
Debt Instrument
Interest rate (percent)	8.312%	8.312%
Debt, effective interest rate (percent)	6.362%	6.362%
Unamortized fair value adjustment, debt	$ 18	$ 19
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member]
Debt Instrument
Number of consecutive years before Company is required to settle deferred interest	5 years
Number of consecutive years Company may defer interest without giving rise to an event of default	10 years
Interest rate (percent)	6.25%	6.25%
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Minimum [Member]
Debt Instrument
Number of occasions the Company may defer payment of interest \| item	1
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from issuance to March 14, 2017 [Member]
Debt Instrument
Interest rate (percent)	6.25%
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from March 15, 2017 until redemption [Member]
Debt Instrument
Percentage of principal amount at which redemption price may be set	100.00%
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from March 15, 2017 until redemption [Member] \| LIBOR [Member]
Debt Instrument
Maturity of LIBOR rate used	three-month LIBOR
Basis spread on variable rate	2.215%
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from March 15, 2037 until redemption [Member]
Debt Instrument
Days in period during which Company will be required to raise proceeds to repay debentures if Company chooses not to redeem the debentures	180 days
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from March 15, 2037 until redemption [Member] \| Minimum [Member]
Debt Instrument
Days before scheduled maturity date that Company's 180-day period must end	10 days
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member] \| Period from March 15, 2037 until redemption [Member] \| Maxiumum [Member]
Debt Instrument
Days before scheduled maturity date that Company's 180-day period must end	15 days

The number of days before the scheduled maturity date that Company's 180-day period must end.

No definition available.

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- Definition

The number of days in the period during which Company will be required to raise proceeds to repay debentures if the Company chooses not to redeem the debentures.

+ References

No definition available.

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- Definition

The reduction to interest expense as a result of amortization of the fair value adjustment.

+ References

No definition available.

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- Definition

The number of junior subordinated debentures that re not fixed-to-floating rate debentures.

+ References

No definition available.

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- Definition

The number of occasions the Company may defer payment of interest.

+ References

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- Definition

The number of separate trusts that issued preferred securities and used proceeds to purchase the Company's subordinated debentures that are not fixed-to-floating rate.

+ References

No definition available.

+ Details

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- Definition

The unamortized adjustment made as a result of the adjustment to fair value for debt assumed in a business combination.

+ References

No definition available.

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- Definition

The number of consecutive years before the Company is required to settle deferred interest.

+ References

No definition available.

+ Details

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- Definition

The number of consecutive years the Company may defer interest without giving rise to an event of default.

+ References

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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.

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ 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

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

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- Definition

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

+ References

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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.

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- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

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