Borrowings (Repurchase Transaction Liability and Market Value of its Underlying Collateral) (Details) - Securities Sold under Agreements to Repurchase [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 282,500	$ 652,229
Assets Sold Under Agreements To Repurchase Interest Rate	1.56%	2.47%
Market value of underlying collateral of a repurchase agreement	$ 301,684	$ 700,498
FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	301,684	575,915
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	75,629
US Treasury Securities [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	0	48,954
Mturity less than 90 Days Member
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		$ 349,729
Assets Sold Under Agreements To Repurchase Interest Rate		3.35%
Market value of underlying collateral of a repurchase agreement		$ 372,778
Mturity less than 90 Days Member \| FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		248,288
Mturity less than 90 Days Member \| Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		75,536
Mturity less than 90 Days Member \| US Treasury Securities [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		48,954
Maturity over 90 days
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 282,500	$ 302,500
Assets Sold Under Agreements To Repurchase Interest Rate	1.56%	1.44%
Market value of underlying collateral of a repurchase agreement	$ 301,684	$ 327,720
Maturity over 90 days \| FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	301,684	327,627
Maturity over 90 days \| Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	0	93
Maturity over 90 days \| US Treasury Securities [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 0	$ 0

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

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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

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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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