Fair Value Measurements on a Recurring Basis (Details) (Recurring Measurements, USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financial Assets
Total assets measured at fair value on a recurring basis	$ 2,544,093us-gaap_AssetsFairValueDisclosure	$ 2,387,428us-gaap_AssetsFairValueDisclosure
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	21,410	16,668
Interest rate swaps
Financial Liabilities
Interest rate swaps	21,410	16,668
U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	52,852	44
U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	78,228	21,945
State and local governments
Financial Assets
Investment securities, available-for-sale	996,146	997,969
Corporate bonds
Financial Assets
Investment securities, available-for-sale	380,310	314,854
Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,036,557	1,052,616
Level 1
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 1 \| Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 1 \| U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 \| U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 \| State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 \| Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 \| Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 2
Financial Assets
Total assets measured at fair value on a recurring basis	2,544,093	2,387,428
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	21,410	16,668
Level 2 \| Interest rate swaps
Financial Liabilities
Interest rate swaps	21,410	16,668
Level 2 \| U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	52,852	44
Level 2 \| U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	78,228	21,945
Level 2 \| State and local governments
Financial Assets
Investment securities, available-for-sale	996,146	997,969
Level 2 \| Corporate bonds
Financial Assets
Investment securities, available-for-sale	380,310	314,854
Level 2 \| Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,036,557	1,052,616
Level 3
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 3 \| Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 3 \| U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 \| U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 \| State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 \| Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 \| Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

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

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

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

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-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

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