Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 7,370,529invest_DerivativeNotionalAmount	$ 6,837,571invest_DerivativeNotionalAmount
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	4,808	4,177
Financial derivatives, at fair value	95,493	84,844
Derivative, Collateral [Abstract]
Collateral Pledged	48,870	46,627
Derivative Asset	4,808us-gaap_DerivativeAssets	4,177us-gaap_DerivativeAssets
Derivative Liability	46,623us-gaap_DerivativeLiabilities	38,217us-gaap_DerivativeLiabilities
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	48,900	46,600
Designated as Hedging Instrument [Member] \| Fair Value Hedging [Member] \| Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	1,014,915	1,000,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	37,478	31,718
Derivative, Weighted-Average Pay Rate	2.46%	2.47%
Derivative, Weighted-Average Receive Rate	0.26%	0.23%
Derivative, Average Remaining Maturity	3 years 9 months 29 days	3 years 11 months 23 days
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	37,000	15,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	751	289
Derivative, Weighted-Average Pay Rate	2.43%	2.43%
Derivative, Weighted-Average Receive Rate	0.57%	0.51%
Derivative, Average Remaining Maturity	8 years 3 months 18 days	6 years 2 months 23 days
Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	1	22
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	244	187
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	476,113	490,183
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	19	537
Financial derivatives, at fair value	56,602	51,224
Derivative, Weighted-Average Pay Rate	4.19%	4.23%
Derivative, Weighted-Average Receive Rate	0.26%	0.23%
Derivative, Average Remaining Maturity	6 years 11 months 26 days	7 years 0 months 18 days
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	4,042,209	3,829,355
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	4,305	3,414
Financial derivatives, at fair value	230	461
Derivative, Weighted-Average Pay Rate	0.15%	0.14%
Derivative, Weighted-Average Receive Rate	0.30%	0.27%
Derivative, Average Remaining Maturity	0 years 6 months 25 days	0 years 6 months 18 days
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap, Receive Fixed Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	632,565	383,565
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	238	1
Financial derivatives, at fair value	277	877
Derivative, Weighted-Average Pay Rate	0.13%	0.12%
Derivative, Weighted-Average Receive Rate	1.02%	1.34%
Derivative, Average Remaining Maturity	2 years 9 months 18 days	3 years 5 months 19 days
Not Designated as Hedging Instrument [Member] \| Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	1,130,000	1,105,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	247	247
Financial derivatives, at fair value	262	406
Derivative, Weighted-Average Pay Rate	0.12%	0.11%
Derivative, Weighted-Average Receive Rate	0.31%	0.31%
Derivative, Average Remaining Maturity	2 years 7 months 10 days	2 years 5 months 1 day
Not Designated as Hedging Instrument [Member] \| Forward Contracts [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	34,827	12,768
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	132	53
Derivative, Weighted-Average Forward Price	$ 101.41	$ 101.00
Not Designated as Hedging Instrument [Member] \| Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,900	1,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 5	$ 3
Derivative, Weighted-Average Forward Price	$ 128.73	$ 126.60

Derivative, Weighted-Average Forward Price

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- Definition

Derivative, Weighted-Average Pay Rate

+ References

No definition available.

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- Definition

Derivative, Weighted-Average Receive Rate

+ References

No definition available.

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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

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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)

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- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other 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 assets not subject to a master netting arrangement and not elected to be offset.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

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-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (c)

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-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 10

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Amount of credit risk valuation adjustment to derivative assets to properly reflect the credit quality of the counterparties.

+ References

No definition available.

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- Definition

Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

+ References

No definition available.

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other 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 assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

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-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

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-Paragraph 4B

-Subparagraph (a),(c)

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-Topic 210

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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.

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- Definition

Fair value, after the 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 not subject to a master netting arrangement and not elected to be offset.

+ References

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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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.15)

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-Name Accounting Standards Codification

-Glossary Collateral

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