| **Debt (Narrative) (Details) - USD ($) $ in Millions**                               | Dec. 31, 2017 | Dec. 31, 2016 |
|--------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Disclosure [Abstract]**                                                       |          |          |
| Federal Home Loan Bank stock                                                         | $ 5           |          |
| Federal Home Loan Bank, advances, general debt obligations, maximum amount available | 111           |          |
| Federal Home Loan Bank, advances, branch of FHLB Bank, amount of advances            | 0             | $ 0           |
| Line of credit, current borrowing capacity                                           | 250           |          |
| Additional borrowing capacity available                                              | 100           |          |
| Line of credit, minimum net worth required for compliance                            | 8,700         |          |
| Line of credit, amount outstanding                                                   | $ 0           | $ 0           |

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| - References
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| - Definition
Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of advances from the Federal Home Loan Bank (FHLB).
+ References
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| - Definition
Amount of maximum advances or credit lines available from the Federal Home Loan Bank (FHLB).
+ References
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| - Definition
Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.
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| - Definition
Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.
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| - Definition
The lowest amount of net worth (total assets minus total liabilities) required by secondary market investors or state-imposed regulatory mandates.
+ References
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