Borrowings (Repurchase Transaction Liability and Market Value of its Underlying Collateral) (Details) - Securities Sold under Agreements to Repurchase [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 998,532	$ 977,816
Assets Sold Under Agreements To Repurchase Interest Rate	2.92%	2.89%
Market value of underlying collateral of a repurchase agreement	$ 1,099,378	$ 1,088,525
FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	1,096,949	1,087,272
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	2,429	1,253
Maturity up to 30 days
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 88,532	$ 52,816
Assets Sold Under Agreements To Repurchase Interest Rate	0.49%	0.39%
Market value of underlying collateral of a repurchase agreement	$ 95,023	$ 56,066
Maturity up to 30 days \| FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	95,023	56,066
Maturity up to 30 days \| Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	0	0
Maturity over 90 days
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 910,000	$ 925,000
Assets Sold Under Agreements To Repurchase Interest Rate	3.16%	2.83%
Market value of underlying collateral of a repurchase agreement	$ 1,004,355	$ 1,032,459
Maturity over 90 days \| FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	1,001,926	1,031,206
Maturity over 90 days \| Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	$ 2,429	$ 1,253

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

-Subparagraph 1

-Article 4

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Market value of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

-Subparagraph 1

-Article 4

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Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

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