Nature of Business and Its Significant Accounting Policies - Loans Held for Sale (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Loans held for sale
Provision for credit losses	$ 1,441	$ 1,175
Johnson Mortgage Company LLC \| Real Estate
Loans held for sale
Receivable settlement period on mortgage loans sold	30 days
Allowance for repurchase of loans sold	$ 0
Provision for credit losses	$ 0
Johnson Mortgage Company LLC \| Real Estate \| Minimum
Loans held for sale
Number of family residences in structures for which mortgages are purchased or refinanced \| item	1
Johnson Mortgage Company LLC \| Real Estate \| Maximum
Loans held for sale
Number of family residences in structures for which mortgages are purchased or refinanced \| item	4

Amount of expense related to estimated loss from loan and lease transactions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

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