Financial Derivatives - Narrative (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Financial derivatives, at fair value	$ 95,493,000	$ 84,844,000
Derivative, Collateral [Abstract]
Collateral Pledged	48,870,000	46,627,000
Additional Collateral, Aggregate Fair Value	47,900,000	46,800,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,370,529,000invest_DerivativeNotionalAmount	6,837,571,000invest_DerivativeNotionalAmount
Excluding netting [Member] \| Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	8,700,000	6,100,000
Financial derivatives, at fair value	105,300,000	99,400,000
Netting [Member] \| Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	300,000	400,000
Financial derivatives, at fair value	96,800,000	93,400,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	300,000	400,000
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	48,900,000	46,600,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0	0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	4,200,000,000	4,000,000,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 7,300,000,000	$ 6,800,000,000

Credit exposure to interest rate swap counterparties

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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- Definition

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

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- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313

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- Definition

The fair value of collateral received by the entity associated with securities borrowed that must be returned if the counterparty fulfills its obligations. It appears on the balance sheet as a liability and has a corresponding asset that appears on the balance sheet called Securities Received as Collateral.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.15)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Collateral

-URI http://asc.fasb.org/extlink&oid=6507830

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