Junior Subordinated Debt (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) Trust Period
Junior Subordinated Debt [Abstract]
Number of statutory business trusts included in the Trusts \| Trust	5
Minimum assets for bank holding companies to be subject to the same capital requirements as insured depository institutions	$ 500,000
CNBF Capital Trust I [Member]
Junior Subordinated Debt [Abstract]
Number of wholly owned Delaware statutory business trusts	1
CNBF Capital Trust I [Member] \| LIBOR [Member]
Junior Subordinated Debt [Abstract]
Issuance Date	Aug. 01, 1999
Trust Preferred Securities Outstanding	$ 18,000
Variable rate basis	3-month LIBOR
Basis spread on variable rate	2.75%
Trust Preferred Debt Owed To Trust	$ 18,720
Final Maturity Date	Aug. 01, 2029
NBT Statutory Trust I [Member] \| LIBOR [Member]
Junior Subordinated Debt [Abstract]
Issuance Date	Nov. 01, 2005
Trust Preferred Securities Outstanding	$ 5,000
Variable rate basis	3-month LIBOR
Basis spread on variable rate	1.40%
Trust Preferred Debt Owed To Trust	$ 5,155
Final Maturity Date	Dec. 01, 2035
NBT Statutory Trust II [Member] \| LIBOR [Member]
Junior Subordinated Debt [Abstract]
Issuance Date	Feb. 01, 2006
Trust Preferred Securities Outstanding	$ 50,000
Variable rate basis	3-month LIBOR
Basis spread on variable rate	1.40%
Trust Preferred Debt Owed To Trust	$ 51,547
Final Maturity Date	Mar. 01, 2036
Alliance Financial Capital Trust I [Member] \| LIBOR [Member]
Junior Subordinated Debt [Abstract]
Issuance Date	Dec. 01, 2003
Trust Preferred Securities Outstanding	$ 10,000
Variable rate basis	3-month LIBOR
Basis spread on variable rate	2.85%
Trust Preferred Debt Owed To Trust	$ 10,310
Final Maturity Date	Jan. 01, 2034
Alliance Financial Capital Trust II [Member] \| LIBOR [Member]
Junior Subordinated Debt [Abstract]
Issuance Date	Sep. 01, 2006
Trust Preferred Securities Outstanding	$ 15,000
Variable rate basis	3-month LIBOR
Basis spread on variable rate	1.65%
Trust Preferred Debt Owed To Trust	$ 15,464
Final Maturity Date	Sep. 01, 2036
Alliance Financial Capital [Member]
Junior Subordinated Debt [Abstract]
Number of wholly owned Delaware statutory business trusts	2
Trusts [Member]
Junior Subordinated Debt [Abstract]
Trust Preferred Securities Outstanding	$ 98,000
Trust Preferred Debt Owed To Trust	101,000
Trust equity method investment	$ 3,200
Debentures period semi-annual deferral periods \| Period	10
Trust preferred securities included in Tier I capital	$ 97,000
NBT Bancorp Inc [Member]
Junior Subordinated Debt [Abstract]
Trust equity method investment	$ 1,000

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

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

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

+ References

No definition available.

+ Details

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Balance Type:	na
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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.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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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.

+ Details

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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+ Details

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- References

No definition available.

+ Details

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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

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Balance Type:	debit
Period Type:	instant

- Details

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