Estimated Fair Values (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Estimated Fair Values [Line Items]
Total investment securities available for sale, fair value	$ 7,045,109us-gaap_AvailableForSaleSecurities	$ 7,171,917us-gaap_AvailableForSaleSecurities
Liability transfers between Level 1 and Level 2	0
Asset transfers between Level 1 and Level 2	0
Liability transfers between Level 2 and Level 1	0
Asset transfers between Level 2 and Level 1	0
Fair Value, Measurements, Nonrecurring
Estimated Fair Values [Line Items]
Liability measured at fair value on nonrecurring basis	$ 0	$ 0
Minimum
Estimated Fair Values [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	10.00%	10.00%
Maximum
Estimated Fair Values [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	14.00%	14.00%

Discounts for Collateral Value Estimates Due to Estimated Holding and Selling Costs (Percent)

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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-SubTopic 10

-Section 45

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Fair value of financial and nonfinancial obligations.

+ References

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-Topic 820

-SubTopic 10

-Section 50

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