Investment Securities - Allowance for Credit Losses AFS (Details) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 0
Credit losses on securities not previously recorded	218
Net increase (decrease) in allowance on previously impaired securities	(322)
Balance, March 31, 2020	269
Impact of adoption of 2016-13
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	373
State and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	0
Credit losses on securities not previously recorded	44
Net increase (decrease) in allowance on previously impaired securities	(322)
Balance, March 31, 2020	95
State and political subdivisions \| Impact of adoption of 2016-13
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	373
Other securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	0
Credit losses on securities not previously recorded	174
Net increase (decrease) in allowance on previously impaired securities	0
Balance, March 31, 2020	174
Other securities \| Impact of adoption of 2016-13
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 0

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 4: 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=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amount of expense for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), for which no credit loss was previously recorded.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotPreviouslyRecorded
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense (reversal of expense) for allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), not intended to be sold and not more likely than not will be required to sell before recovery of amortized cost basis, with previously recorded allowance.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLossNotToSellBeforeRecoveryCreditLossPreviouslyRecordedExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Details

Name:	sfnc_CumulativeEffectPeriodOfAdoptionAxis=sfnc_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=sfnc_OtherSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: