Loans and Allowance for Credit Losses (Finance Receivable, Nonaccrual) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	$ 150,831		$ 205,885
Recorded Investment, With no specific allowance recorded	50,282		69,195
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	1,300	$ 3,000
Residential Portfolio Segment [Member]
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	19,824		28,553
Recorded Investment, With no specific allowance recorded	0		1,684
Multi-family
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	10,725		10,865
Recorded Investment, With no specific allowance recorded	10,725		10,865
Commercial real estate, Non-owner occupied
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	25,667		39,251
Recorded Investment, With no specific allowance recorded	11,069		20,929
Construction and land
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	5,090		5,164
Recorded Investment, With no specific allowance recorded	4,301		4,369
Commercial real estate, Owner occupied
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	21,079		20,453
Recorded Investment, With no specific allowance recorded	5,707		4,457
Commercial and industrial
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	40,222		68,720
Recorded Investment, With no specific allowance recorded	16,176		10,083
Bridge - franchise finance
Financing Receivable Investment Past Due [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	28,224		32,879
Recorded Investment, With no specific allowance recorded	$ 2,304		$ 16,808

Amount of financing receivable on nonaccrual status with no allowance for credit loss.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919258-210447

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5111-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bku_CommercialRealEstateNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bku_ConstructionLandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bku_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bku_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bku_BridgefranchisefinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: