Investment Securities Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)
Available-for-sale securities pledged as collateral	$ 1,800,000
Available-for-sale securities, amortized cost basis	2,575,719	$ 2,594,255
Securities available for sale	2,547,956	2,559,227
Auction rate securities
Available-for-sale securities, amortized cost basis	107,410	107,215
Securities available for sale	98,668	$ 97,256
Single-issuer trust preferred securities
Unrealized gain (loss) on securities	$ (600)
Number of securities \| Security	18
Single-issuer trust preferred securities \| External Credit Rating, Rated Below Investment Grade
Available-for-sale securities, amortized cost basis	$ 4,900
Securities available for sale	$ 4,700
Number of trust preferred securities	4
Single-issuer trust preferred securities \| Not Rated by an External Agency
Available-for-sale securities, amortized cost basis	$ 3,800
Securities available for sale	$ 3,100

Number Of Securities

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Number of Trust Preferred Securities Below Investment Grade.

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- Definition

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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
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Amount of unrealized gain (loss) recognized in the income statement on unsold other securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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-Section 45
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