Borrowings and Federal Home Loan Bank Advances (Narrative) (Details)	12 Months Ended
Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Short Term Debt [Line Items]
Uncommitted secured lines of credit	$ 1,000,000,000
Number of banks \| item	6
Daily borrowings under our uncommitted secured lines	$ 444,400,000
Compensating balances	0
Trading securities pledged	2,000,000,000	$ 1,700,000,000
Federal home loan advances, floating-rate	745,000,000
Revolving Credit Facility
Short Term Debt [Line Items]
Committed revolving credit facility	$ 200,000,000
Revolving credit facility expiration date	2020-03
LIBOR rate	2.00%
Outstanding on our revolving credit facility	$ 0
Company Owned Securities [Member]
Short Term Debt [Line Items]
Uncommitted secured lines of credit	256,000,000
Trading securities pledged	$ 292,700,000
Federal Home Loan Bank advances [Member]
Short Term Debt [Line Items]
Weighted average interest rate on borrowings	1.07%

Daily borrowings under credit arrangements.

No definition available.

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- Definition

Line of credit facility expiration month and year.

+ References

No definition available.

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- Definition

Number of banks with which entity has committed revolving credit facility.

+ References

No definition available.

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- Definition

Uncommitted secured lines of credit.

+ References

No definition available.

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- Definition

Weighted average interest rate on borrowings.

+ References

No definition available.

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- Definition

The amount of a specific compensating balance arrangement that is maintained under an agreement for a bank loan or future credit availability.

+ References

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- Definition

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

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No definition available.

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- Definition

Par amount of advances made and reported by Federal Home Loan Bank (FHLBank) with floating interest rate.

+ References

No definition available.

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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.

+ 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))
-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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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.

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- Definition

The fair value of trading securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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