Debt Obligations - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument
Debt Instrument, Fair Value Disclosure	$ 830	$ 1,140
Long term debt, amount outstanding	818	1,141
Note payable to related party	113	0
Junior Subordinated Notes	54	54
Unamortized Net Premiums And Fair Value Adjustments	$ 28	$ 51
3.26% Junior Subordinated Notes due November 1, 2066 [Member]
Debt Instrument
Debt Instrument, Interest Rate, Stated Percentage	3.0175%
Long-term Debt, Weighted Average Interest Rate, at Point in Time	4.39428%	3.903%
Panhandle [Member]
Debt Instrument
Junior Subordinated Notes	$ 600
Senior Notes Due 2029 [Member]
Debt Instrument
Senior Notes	$ 66	$ 66
Debt Instrument, Interest Rate, Stated Percentage	7.00%
7.60% Senior Notes, due February 1, 2024 [Member]
Debt Instrument
Senior Notes	$ 82	82
Debt Instrument, Interest Rate, Stated Percentage	7.60%
8.25% Senior Notes, due November 14, 2029 [Member]
Debt Instrument
Senior Notes	$ 33	$ 33
Debt Instrument, Interest Rate, Stated Percentage	8.25%

Unamortized Net Premiums And Fair Value Adjustments

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
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- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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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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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

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- Definition

The amount for notes payable (written promise to pay), payable to related parties, which are due after one year (or one business cycle).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

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