Investment Securities - Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	$ 4,316,977	$ 3,688,091
Gross Unrealized Gains	223,895	112,651
Gross Unrealized Losses	(8,258)	(3,555)
Securities available-for-sale	4,532,614	3,797,187
Government agency and government-sponsored enterprise pass through securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,129,060	1,083,182
Gross Unrealized Gains	75,309	25,579
Gross Unrealized Losses	(54)	(537)
Securities available-for-sale	1,204,315	1,108,224
Government agency and government-sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,111,235	1,112,573
Gross Unrealized Gains	54,900	24,403
Gross Unrealized Losses	(263)	(579)
Securities available-for-sale	1,165,872	1,136,397
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,133,811	691,647
Gross Unrealized Gains	70,758	43,851
Gross Unrealized Losses	(1,539)	(339)
Securities available-for-sale	1,203,030	735,159
Agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	233,513	294,606
Gross Unrealized Gains	13,018	10,593
Gross Unrealized Losses	(33)	(1)
Securities available-for-sale	246,498	305,198
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	236,081	216,133
Gross Unrealized Gains	554	320
Gross Unrealized Losses	(2,532)	(1,670)
Securities available-for-sale	234,103	214,783
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	138,966	124,134
Gross Unrealized Gains	0	25
Gross Unrealized Losses	(3,633)	(403)
Securities available-for-sale	135,333	123,756
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	115,101	96,066
Gross Unrealized Gains	6,089	3,430
Gross Unrealized Losses	(28)	(13)
Securities available-for-sale	121,162	99,483
SBA asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	40,515	47,765
Gross Unrealized Gains	2,194	506
Gross Unrealized Losses	(25)	(13)
Securities available-for-sale	42,684	48,258
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	173,707	17,000
Gross Unrealized Gains	726	3,748
Gross Unrealized Losses	(151)	0
Securities available-for-sale	174,282	20,748
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	4,988	4,985
Gross Unrealized Gains	347	196
Gross Unrealized Losses	0	0
Securities available-for-sale	$ 5,335	$ 5,181

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=pacw_GovernmentagencyandgovernmentsponsoredenterprisecollateralizedmortgageobligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=pacw_PrivateLabelCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=pacw_SBAassetbackedsecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: