Junior Subordinated Debt (Details) (USD $)	12 Months Ended
Dec. 31, 2014
Debt Instrument [Line Items]
Number of statutory business trusts included in the Trusts	5
Minimum assets for bank holding companies to be subject to the same capital requirements as insured depository institutions	$ 500,000,000
CNBF Capital Trust I Debenture [Member]
Debt Instrument [Line Items]
Issuance Date	Aug. 01, 1999
Trust Preferred Securities Outstanding	18,000,000
Interest Rate	3-month LIBOR plus 2.75%
Trust Preferred Debt Owed To Trust	18,720,000
Final Maturity Date	Aug. 01, 2029
Number of wholly owned Delaware statutory business trusts	1
NBT Statutory Trust I Debenture [Member]
Debt Instrument [Line Items]
Issuance Date	Nov. 01, 2005
Trust Preferred Securities Outstanding	5,000,000
Interest Rate	3-month LIBOR plus 1.40%
Trust Preferred Debt Owed To Trust	5,155,000
Final Maturity Date	Dec. 01, 2035
NBT Statutory Trust II Debenture [Member]
Debt Instrument [Line Items]
Issuance Date	Feb. 01, 2006
Trust Preferred Securities Outstanding	50,000,000
Interest Rate	3-month LIBOR plus 1.40%
Trust Preferred Debt Owed To Trust	51,547,000
Final Maturity Date	Mar. 01, 2036
Alliance Financial Capital Trust I [Member]
Debt Instrument [Line Items]
Issuance Date	Dec. 01, 2003
Trust Preferred Securities Outstanding	10,000,000
Interest Rate	3-month LIBOR plus 2.85%
Trust Preferred Debt Owed To Trust	10,310,000
Final Maturity Date	Jan. 01, 2034
Alliance Financial Capital Trust II [Member]
Debt Instrument [Line Items]
Issuance Date	Sep. 01, 2006
Trust Preferred Securities Outstanding	15,000,000
Interest Rate	3-month LIBOR plus 1.65%
Trust Preferred Debt Owed To Trust	15,464,000
Final Maturity Date	Sep. 01, 2036
NBT Bancorp Inc [Member]
Debt Instrument [Line Items]
Trust equity method investment	1,000,000
Alliance Financial Capital [Member]
Debt Instrument [Line Items]
Number of wholly owned Delaware statutory business trusts	2
Trusts [Member]
Debt Instrument [Line Items]
Trust Preferred Securities Outstanding	98,000,000
Trust Preferred Debt Owed To Trust	101,000,000
Trust equity method investment	3,200,000
Debentures period semi-annual deferral periods	10
Trust preferred securities included in Tier I capital	$ 97,000,000

Amount of trust preferred debt owed to the trust at the balance sheet date.

No definition available.

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- Definition

For an issue of Subordinated Debentures, maximum distribution deferral period (payments of interest).

+ References

No definition available.

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- Definition

The minimum assets for bank holding companies to be subject to the same capital requirements as insured depository institutions under the Dodd-Frank Act.

+ References

No definition available.

+ Details

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- Definition

The number of statutory business trusts issued junior subordinated deferrable interest debentures.

+ References

No definition available.

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- Definition

Represents the number of statutory business trusts included in the Trusts.

+ References

No definition available.

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- Definition

Tier 1 Risk Based Capital as defined in the regulations that includes only trust preferred securities.

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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-SubTopic 30

-Section 55

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- Definition

Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

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- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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- Definition

Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ 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 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB

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