Borrowings (Repurchase Transaction Liability and Market Value of its Underlying Collateral) (Details) - Securities Sold under Agreements to Repurchase [Member] - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		$ 932,500,000
Assets Sold Under Agreements To Repurchase Interest Rate		3.10%
Market value of underlying collateral of a repurchase agreement		$ 1,021,370,000
FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement		1,006,659,000
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		2,131,000
US Treasury Securities [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement		12,580,000
Mturity less than 90 Days Member
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability		$ 30,000,000
Assets Sold Under Agreements To Repurchase Interest Rate		0.70%
Market value of underlying collateral of a repurchase agreement		$ 31,961,000
Mturity less than 90 Days Member \| FNMA and FHLMC [Member]
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Interest Rate		0.70%
Market value of underlying collateral of a repurchase agreement		$ 31,961,000
Maturity over 90 days
Assets Sold Under Agreements To Repurchase Line Items
Assets Sold Under Agreements To Repurchase Repurchase Liability	$ 624,500,000	$ 902,500,000
Assets Sold Under Agreements To Repurchase Interest Rate	2.69%	3.18%
Market value of underlying collateral of a repurchase agreement	$ 668,689,000	$ 989,409,000
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	646,889,000	974,698,000
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	1,579,000	2,131,000
Maturity over 90 days \| US Treasury Securities [Member]
Assets Sold Under Agreements To Repurchase Line Items
Market value of underlying collateral of a repurchase agreement	$ 20,221,000	$ 12,580,000

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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