Remaining unamortized costs as of the balance sheet date that were essential to acquiring the lease, and that would not otherwise have been incurred without the lease agreement, including evaluating the lessee's credit condition, guarantees, and collateral and costs incurred negotiating, processing, and closing the lease agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40246-112709

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45377-112738

+ Details

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

+ Details

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

- Details

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

- Details

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

- 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=us-gaap_CommercialAndIndustrialSectorMember
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:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_CollateralAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_EquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Loans and Allowance for Credit Losses (Impaired Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	$ 148,235	$ 136,226
Financing Receivable, before Allowance for Credit Loss			$ 111,854
Recorded Investment, With no specific allowance recorded			52,962
Recorded Investment, Total				$ 244,905
UPB, Total				245,029
Related Specific Allowance, Impaired loans			196,216	204,788
Capital Leases, Net Investment in Direct Financing Leases, Initial Direct Costs			687,000	733,000
1-4 single family residential
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	4,685	4,639
Recorded Investment, With no specific allowance recorded				992
Recorded Investment, With a specific allowance recorded				2,697
UPB, With no specific allowance recorded				989
UPB, With a specific allowance recorded				2,652
US Government Agency Insured Loans [Member]
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	19,865	10,885
Recorded Investment, With no specific allowance recorded				53,428
UPB, With no specific allowance recorded				53,350
Residential and Other Consumer
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	24,550	15,524
Financing Receivable, before Allowance for Credit Loss			3,339
Recorded Investment, With no specific allowance recorded			2,683
Recorded Investment, Total				57,117
UPB, Total				56,991
Related Specific Allowance, Impaired loans			8,031	18,894
Multi-family
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	21,370	23,955
Recorded Investment, With no specific allowance recorded			0	6,138
UPB, With no specific allowance recorded				6,169
Related Specific Allowance, Impaired loans			0	6,138
Commercial real estate, Non-owner occupied
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	11,938	10,628
Financing Receivable, before Allowance for Credit Loss			40,348
Recorded Investment, With no specific allowance recorded			27,002	38,345
Recorded Investment, With a specific allowance recorded				2,522
UPB, With no specific allowance recorded				38,450
UPB, With a specific allowance recorded				2,509
Related Specific Allowance, Impaired loans			54,944	40,097
Construction and land
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	6,188	7,854
Financing Receivable, before Allowance for Credit Loss			4,432
Recorded Investment, With no specific allowance recorded			4,086	3,191
UPB, With no specific allowance recorded				3,155
Related Specific Allowance, Impaired loans			4,432	3,191
Commercial real estate, Owner occupied
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	3,406	3,526
Financing Receivable, before Allowance for Credit Loss			15,107
Recorded Investment, With no specific allowance recorded			4,835	17,419
UPB, With no specific allowance recorded				17,488
Related Specific Allowance, Impaired loans			23,953	27,141
Commercial and industrial
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	27,297	27,879
Financing Receivable, before Allowance for Credit Loss			29,506
Recorded Investment, With no specific allowance recorded			11,898	10,585
Recorded Investment, With a specific allowance recorded				63,531
UPB, With no specific allowance recorded				10,574
UPB, With a specific allowance recorded				63,709
Related Specific Allowance, Impaired loans			72,094	74,757
Bridge - franchise finance [Member]
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			19,122
Recorded Investment, With no specific allowance recorded			2,458	4,115
Recorded Investment, With a specific allowance recorded				21,011
UPB, With no specific allowance recorded				4,117
UPB, With a specific allowance recorded				21,050
Related Specific Allowance, Impaired loans			32,762	13,631
Bridge - equipment finance [Member]
Financing Receivable Impaired [Line Items]
Recorded Investment, With no specific allowance recorded			0	6,807
Recorded Investment, With a specific allowance recorded				14,124
UPB, With no specific allowance recorded				6,793
UPB, With a specific allowance recorded				14,024
Related Specific Allowance, Impaired loans			0	20,939
Commercial Portfolio Segment [Member]
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	123,685	120,702
Financing Receivable, before Allowance for Credit Loss			108,515
Recorded Investment, Total				187,788
UPB, Total				188,038
Commercial Finance Subsidiaries [Member]
Financing Receivable Impaired [Line Items]
Average recorded investment in impaired loans	$ 53,486	$ 46,860
Residential Real Estate [Member] \| Residential and Other Consumer
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			3,331
Commercial Real Estate [Member] \| Commercial real estate, Non-owner occupied
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			40,288
Commercial Real Estate [Member] \| Construction and land
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			4,432
Commercial Real Estate [Member] \| Commercial real estate, Owner occupied
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			15,107
Commercial Real Estate [Member] \| Commercial and industrial
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			17,506
Commercial Real Estate [Member] \| Commercial Portfolio Segment [Member]
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			90,057
Equipment [Member] \| Bridge - franchise finance [Member]
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			12,724
Secured Debt [Member]
Financing Receivable Impaired [Line Items]
Financing Receivable, before Allowance for Credit Loss			$ 93,388
Specific Valuation Allowance [Member]
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				20,490
Specific Valuation Allowance [Member] \| 1-4 single family residential
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				9
Specific Valuation Allowance [Member] \| Residential and Other Consumer
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				9
Specific Valuation Allowance [Member] \| Commercial real estate, Owner occupied
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				401
Specific Valuation Allowance [Member] \| Commercial and industrial
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				13,992
Specific Valuation Allowance [Member] \| Bridge - franchise finance [Member]
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				2,953
Specific Valuation Allowance [Member] \| Bridge - equipment finance [Member]
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				3,135
Specific Valuation Allowance [Member] \| Commercial Portfolio Segment [Member]
Financing Receivable Impaired [Line Items]
Related Specific Allowance, Impaired loans				$ 20,481