Fair Value Measurements - Summary of carrying amounts and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Securities AFS	$ 535,351	$ 449,302
Carrying Amount
Financial assets:
Cash and cash equivalents	853,564	182,059
Certificates of deposit in other banks	32,969	19,305
Securities AFS	535,351	449,302
Other investments, including equity securities	26,636	24,072
Loans held for sale	8,384	2,706
Loans, net	2,877,405	2,559,779
BOLI	82,905	78,140
MSR asset	8,685	5,919
Financial liabilities:
Deposits	3,712,808	2,954,453
Long-term borrowings	405,826	67,629
Estimated Fair Value
Financial assets:
Cash and cash equivalents	853,564	182,059
Certificates of deposit in other banks	35,112	19,310
Securities AFS	535,351	449,302
Other investments, including equity securities	26,636	24,072
Loans held for sale	8,712	2,753
Loans, net	2,962,975	2,593,110
BOLI	82,905	78,140
MSR asset	10,332	8,420
Financial liabilities:
Deposits	3,720,344	2,956,229
Long-term borrowings	405,787	66,816
Estimated Fair Value \| Level 1
Financial assets:
Cash and cash equivalents	853,564	182,059
Certificates of deposit in other banks	0	0
Securities AFS	0	0
Other investments, including equity securities	3,124	3,375
Loans held for sale	0	0
Loans, net	0	0
BOLI	82,905	78,140
MSR asset	0	0
Financial liabilities:
Deposits	0	0
Long-term borrowings	0	0
Estimated Fair Value \| Level 2
Financial assets:
Cash and cash equivalents	0	0
Certificates of deposit in other banks	35,112	19,310
Securities AFS	532,221	446,172
Other investments, including equity securities	19,472	16,759
Loans held for sale	8,712	2,753
Loans, net	0	0
BOLI	0	0
MSR asset	0	0
Financial liabilities:
Deposits	0	0
Long-term borrowings	363,471	25,075
Estimated Fair Value \| Level 3
Financial assets:
Cash and cash equivalents	0	0
Certificates of deposit in other banks	0	0
Securities AFS	3,130	3,130
Other investments, including equity securities	4,040	3,938
Loans held for sale	0	0
Loans, net	2,962,975	2,593,110
BOLI	0	0
MSR asset	10,332	8,420
Financial liabilities:
Deposits	3,720,344	2,956,229
Long-term borrowings	$ 42,316	$ 41,741

This element represent deposits made by entity with other banks.

No definition available.

Name:	ncbs_CertificatesOfDepositFairValueDisclosure
Namespace Prefix:	ncbs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents MSR asset.

+ References

No definition available.

+ Details

Name:	ncbs_MsrAsset
Namespace Prefix:	ncbs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

Name:	ncbs_OtherInvestmentFairValueDisclosure
Namespace Prefix:	ncbs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of the amount that could be realized under a life insurance contract or contracts owned by the entity, commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

+ References

No definition available.

+ Details

Name:	us-gaap_CashSurrenderValueFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

Name:	us-gaap_DepositsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: