Organization Organization (Policies)	9 Months Ended
Organization Organization (Policies)	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Accounting Policies	Significant Accounting Policies Our accounting policies are described in Note 1. Nature of Operations and Summary of Significant Accounting Policies, of our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2017 as filed with the Securities and Exchange Commission ("Form 10-K"). Updates to our significant accounting policies described below reflect the impact of the adoption of ASU 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities" and ASU 2018-03, “Technical Corrections and Improvements to Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.” Investment Securities Our significant accounting policy for investment securities applied to both debt and equity securities in prior periods. Effective January 1, 2018, upon the adoption of ASUs 2016-01 and 2018-03, our significant accounting policy for investment securities applies only to debt securities. Equity Investments Investments in common or preferred stock that are not publicly traded and certain investments in limited partnerships are considered equity investments that do not have a readily determinable fair value. If we have the ability to significantly influence the operating and financial policies of the investee, the investment is accounted for pursuant to the equity method of accounting. This is generally presumed to exist when we own between 20% and 50% of a corporation, or when we own greater than 5% of a limited partnership or similarly structured entity. Our equity investment carrying values are included in other assets and our share of earnings and losses in equity method investees is included in "Noninterest income - other" on the condensed consolidated statements of earnings. Prior to January 1, 2018, if we did not have significant influence over the investee, the cost method was used to account for the equity interest. Effective January 1, 2018 with the adoption of ASU 2016-01, our accounting treatment for equity investments differs for those with and without readily determinable fair values. Equity investments with readily determinable fair values are recorded at fair value with changes in fair value recorded in “Noninterest income - other.” For equity investments without readily determinable fair values we have elected the “measurement alternative,” and therefore carry these investments at cost, less impairment (if any), plus or minus changes in observable prices. On a quarterly basis, we review our equity investments without readily determinable fair values for impairment. We consider a number of qualitative factors such as whether there is a significant deterioration in earnings performance, credit rating, asset quality, or business prospects of the investee in determining if impairment exists. If the investment is considered impaired, an impairment loss equal to the amount by which the carrying value exceeds its fair value is recorded through a charge to earnings. The impairment loss may be reversed in a subsequent period if there are observable transactions for the identical or similar investment of the same issuer at a higher amount than the carrying amount that was established when the impairment was recognized. Impairment as well as upward or downward adjustments resulting from observable price changes in orderly transactions for identical or similar investments are included in “Noninterest income - other.” Realized gains or losses resulting from the sale of equity investments are calculated using the specific identification method and are included in "Noninterest income - other." Comprehensive Income Comprehensive income consists of net earnings and net unrealized gains (losses) on debt securities available‑for‑sale, net, and is presented in the consolidated statements of comprehensive income.
Recently Issued Accounting Standards	ORGANIZATION PacWest Bancorp, a Delaware corporation, is a bank holding company registered under the BHCA, with our corporate headquarters located in Beverly Hills, California. Our principal business is to serve as the holding company for our wholly-owned subsidiary, Pacific Western Bank. References to "Pacific Western" or the "Bank" refer to Pacific Western Bank together with its wholly-owned subsidiaries. References to "we," "us," or the "Company" refer to PacWest Bancorp together with its subsidiaries on a consolidated basis. When we refer to "PacWest" or to the "holding company," we are referring to PacWest Bancorp, the parent company, on a stand-alone basis. We are focused on relationship-based business banking to small, middle-market and venture-backed businesses nationwide. At September 30, 2018, the Bank offers a broad range of loan and lease and deposit products and services through 74 full-service branches located throughout the State of California, one branch located in Durham, North Carolina, and numerous loan production offices located in cities across the country. Community Banking provides lending and comprehensive deposit and treasury management services to small and medium-sized businesses conducted primarily through our California-based branch offices. We offer additional products and services through our National Lending and Venture Banking groups. National Lending provides asset-based, equipment, real estate, and security cash flow loans and treasury management services to established middle-market businesses on a national basis. Venture Banking offers a comprehensive suite of financial services focused on entrepreneurial businesses and their venture capital and private equity investors, with offices located in key innovation hubs across the United States. In addition, we provide investment advisory and asset management services to select clients through Square 1 Asset Management, Inc., a wholly-owned subsidiary of the Bank and a SEC-registered investment adviser. We generate our revenue primarily from interest received on loans and leases and, to a lesser extent, from interest received on investment securities, and fees received in connection with deposit services, extending credit and other services offered, including foreign exchange services. Our major operating expenses are compensation, occupancy, general operating expenses, and the interest paid by the Bank on deposits and borrowings. We have completed 29 acquisitions from May 1, 2000 through September 30, 2018. Our acquisitions have been accounted for using the acquisition method of accounting and, accordingly, the operating results of the acquired entities have been included in the consolidated financial statements from their respective acquisition dates. See Note 3. Acquisitions, for more information about the CUB acquisition. El Dorado Savings Bank Merger Announcement On September 11, 2018, PacWest entered into a definitive agreement and plan of merger (the “Agreement”) whereby PacWest will acquire El Dorado Savings Bank, F.S.B. (“El Dorado”) in a transaction valued at approximately $466.7 million. El Dorado, headquartered in Placerville, California, is a federally chartered savings bank founded in 1958, with approximately $2.2 billion in assets and 35 branches located primarily in eight Northern California counties and two Northern Nevada counties. In connection with the transaction, El Dorado will be merged into the Bank. The transaction, which was approved by the PacWest and El Dorado boards of directors, is expected to close in the first quarter of 2019 and is subject to customary closing conditions, including obtaining approval by bank regulatory authorities and El Dorado’s shareholders. Significant Accounting Policies Our accounting policies are described in Note 1. Nature of Operations and Summary of Significant Accounting Policies, of our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2017 as filed with the Securities and Exchange Commission ("Form 10-K"). Updates to our significant accounting policies described below reflect the impact of the adoption of ASU 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities" and ASU 2018-03, “Technical Corrections and Improvements to Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.” Investment Securities Our significant accounting policy for investment securities applied to both debt and equity securities in prior periods. Effective January 1, 2018, upon the adoption of ASUs 2016-01 and 2018-03, our significant accounting policy for investment securities applies only to debt securities. Equity Investments Investments in common or preferred stock that are not publicly traded and certain investments in limited partnerships are considered equity investments that do not have a readily determinable fair value. If we have the ability to significantly influence the operating and financial policies of the investee, the investment is accounted for pursuant to the equity method of accounting. This is generally presumed to exist when we own between 20% and 50% of a corporation, or when we own greater than 5% of a limited partnership or similarly structured entity. Our equity investment carrying values are included in other assets and our share of earnings and losses in equity method investees is included in "Noninterest income - other" on the condensed consolidated statements of earnings. Prior to January 1, 2018, if we did not have significant influence over the investee, the cost method was used to account for the equity interest. Effective January 1, 2018 with the adoption of ASU 2016-01, our accounting treatment for equity investments differs for those with and without readily determinable fair values. Equity investments with readily determinable fair values are recorded at fair value with changes in fair value recorded in “Noninterest income - other.” For equity investments without readily determinable fair values we have elected the “measurement alternative,” and therefore carry these investments at cost, less impairment (if any), plus or minus changes in observable prices. On a quarterly basis, we review our equity investments without readily determinable fair values for impairment. We consider a number of qualitative factors such as whether there is a significant deterioration in earnings performance, credit rating, asset quality, or business prospects of the investee in determining if impairment exists. If the investment is considered impaired, an impairment loss equal to the amount by which the carrying value exceeds its fair value is recorded through a charge to earnings. The impairment loss may be reversed in a subsequent period if there are observable transactions for the identical or similar investment of the same issuer at a higher amount than the carrying amount that was established when the impairment was recognized. Impairment as well as upward or downward adjustments resulting from observable price changes in orderly transactions for identical or similar investments are included in “Noninterest income - other.” Realized gains or losses resulting from the sale of equity investments are calculated using the specific identification method and are included in "Noninterest income - other." Comprehensive Income Comprehensive income consists of net earnings and net unrealized gains (losses) on debt securities available‑for‑sale, net, and is presented in the consolidated statements of comprehensive income. Accounting Standards Adopted in 2018 Effective January 1, 2018, the Company adopted ASU 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities" and ASU 2018-03, “Technical Corrections and Improvements to Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.” ASU 2016-01 contained a number of changes which are applicable to the Company including the following: (1) requires equity investments to be measured at fair value with changes in fair value recognized in net income; (2) allows equity investments without readily determinable fair values to be measured at cost less impairment, if any, plus or minus changes in observable prices (referred to as the "measurement alternative"); and (3) changes certain presentation and disclosure requirements for financial instruments, including using the exit price notion when measuring the fair value of financial instruments (see Note 11. Fair Value Measurements). ASU 2018-03 also clarified certain aspects of the guidance issued in ASU 2016-01, including requiring a prospective transition approach for equity investments without readily determinable fair value in which the measurement alternative is applied. ASU 2016-01 does not apply to investments accounted for using the equity method, investments in consolidated subsidiaries, FHLB stock, and investments in low income housing tax credit projects. Upon adoption of ASU 2016-01, the Company recorded a transition adjustment to reclassify $529,000 in net unrealized gains from accumulated other comprehensive income ("AOCI") to retained earnings. The ASU also eliminated the requirement to classify equity investments into different categories such as “Available-for-sale.” The adoption of this ASU may result in more earnings volatility as changes in fair value of certain equity investments will now be recorded in the statement of earnings as opposed to AOCI. Effective January 1, 2018, the Company early-adopted ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." The TCJA required deferred tax assets and liabilities to be re-measured at its enactment date for the effect of the change in the federal corporate tax rate. This process resulted in "stranded tax effects" in AOCI for deferred tax asset or liabilities which were established with an offsetting amount in AOCI. ASU 2018-02 allows for a reclassification of the stranded tax effects resulting from the enactment of the TCJA from AOCI to retained earnings. The Company elected to reclassify its stranded tax effects of $6.665 million from AOCI to retained earnings effective January 1, 2018, while no other income tax effects related to the application of the TCJA were reclassified. Effective January 1, 2018, the Company adopted ASU 2014-09, "Revenue Recognition (Topic 606): Revenue from Contracts with Customers." ASU 2014-09 supersedes Topic 605, "Revenue Recognition" and requires an entity to recognize revenue at an amount that reflects the consideration to which it expects to be entitled to in exchange for the transfer of promised goods or services to customers. Substantially all of the Company's revenue is interest income on loans, investment securities, and deposits at other financial institutions which are specifically outside the scope of ASU 2014-09. ASU 2014-09 applies primarily to certain noninterest income items in the Company's condensed consolidated statement of earnings. The Company adopted ASU 2014-09 as of January 1, 2018 using the cumulative effect transition method, which resulted in no adjustment to retained earnings and no material impact on the Company's consolidated financial position, results of operations, or cash flows. The Company did make minor changes to accounting operations and internal controls as part of adopting this new standard. See Note 13. Revenue From Contracts With Customers for further details. Effective January 1, 2018, the Company adopted ASU 2016-15, "Classification of Certain Cash Receipts and Cash Payments." Upon adoption, the Company applied the retrospective transition method to each period presented. ASU 2016-15 addressed eight issues related to the statement of cash flows, the most relevant to the Company being the classification of proceeds from the settlement of BOLI policies. As the Company classified proceeds from the settlement of BOLI policies in the manner required by ASU 2016-15 in the prior periods presented, there was no change to the Company's consolidated financial position, results of operations, or cash flows for both current and prior periods upon adoption. Effective January 1, 2018, the Company adopted ASU 2016-18, "Statement of Cash Flows (Topic 230): Restricted Cash." Upon adoption, the Company applied the retrospective transition method to each period presented. As the Company does not present restricted cash as a separate line in the statement of financial position, there is no change to the presentation of cash on the statement of cash flows. The nature and amount of our restricted cash is shown in Note 2. Restricted Cash Balances. Effective January 1, 2018, the Company adopted ASU 2017-01, "Business Combinations (Topic 805): Clarifying the Definition of a Business." ASU 2017-01 provides a new framework for determining whether transactions should be accounted for as acquisitions of assets or businesses. The Company had no acquisitions or purchases of components of a business in the first three quarters of 2018, thus, the impact of adopting the new standard had no impact on the Company's consolidated financial position, results of operations, or cash flows. Effective January 1, 2018, the Company adopted ASU 2017-09, "Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting." ASU 2017-09 provided clarification of what constitutes a modification of a share-based payment award. The Company did not modify any share-based payment awards in the first three quarters of 2018, thus, the impact of adopting the new standard had no impact on the Company's consolidated financial position, results of operations, or cash flows. RECENTLY ISSUED ACCOUNTING STANDARDS In February 2016, the FASB issued ASU 2016-02, "Leases (Topic 842)," which, among other things, requires lessees to recognize most leases on-balance sheet, which will result in an increase in their reported assets and liabilities. Lessor accounting remains substantially similar to current U.S. GAAP. ASU 2016-02 supersedes Topic 840, Leases, and is effective for annual and interim periods in fiscal years beginning after December 15, 2018. There have been further amendments, including practical expedients, with the issuance of ASU 2018-01, “Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842” in January 2018, ASU 2018-10, “Codification Improvements to Topic 842, Leases” in July 2018, and ASU 2018-11, "Leases (Topic 842): Targeted Improvements" in July 2018. The amendments in ASU 2018-11 provide an optional transition method when adopting Topic 842, which allows companies to elect not to adjust their comparative period financial information and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption, effectively applying the requirements of the new standard prospectively. The Company will adopt the standard effective January 1, 2019 and will elect the optional transition method and recognize a cumulative-effect adjustment to retained earnings upon adoption. In addition, the Company will be electing a number of practical expedients permitted under the new standard including carrying forward the historical lease classification and accounting for non-lease and lease components together as a single lease component, and not recognizing a right-of-use asset and lease liability for short-term leases. The primary impact of the new standard to the Company relates to leased branches and office space which are currently accounted for as operating leases. The Company is on track with its implementation plan which includes a new software solution and changes to our processes, procedures, and internal controls. While the ultimate quantitative impact of adoption will not be known until adoption, based on the lease population as of September 30, 2018, the Company anticipates recording lease right-of-use assets and liabilities of approximately $120 to $140 million on its consolidated balance sheet, with no material impact to its consolidated statements of earnings. However, the ultimate impact of adopting ASU 2016-02 will depend on the Company’s lease population and discount rates as of the adoption date. From a lessor perspective, the Company anticipates recognizing more sales-type leases that are currently accounted for as direct financing leases. The change in the definition of initial direct costs to include only incremental direct costs will also result in an acceleration of certain operating costs. Given the limited changes to lessor accounting, the Company does not expect material changes to its consolidated financial statements for the lessor accounting changes. In June 2016, the FASB issued ASU 2016-13, "Measurement of Credit Losses on Financial Instruments," which significantly changes the way entities recognize credit losses and impairment of financial assets recorded at amortized cost. Currently, the credit loss and impairment model for loans and leases is based on incurred losses, and investments are recognized as impaired when there is no longer an assumption that future cash flows will be collected in full under the originally contracted terms. Under the new current expected credit loss ("CECL") model, the standard requires immediate recognition of estimated credit losses expected to occur over the remaining life of the asset. The forward-looking concept of CECL to estimate future credit losses will broaden the range of data to consider including past and current events and conditions along with reasonable and supportable forecasts that may affect expected collectability. The new standard will add new disclosure requirements and impact the Company’s processes and internal controls over financial reporting. The Company has established a multidisciplinary project team and implementation plan, selected a software solution, developed a conceptual framework, and is engaged in the implementation phase of the project. The Company, with the assistance of a third party adviser, continues to work on: (1) developing a new expected loss model with supportable assumptions, (2) identifying data, reporting, and drafting future disclosures, (3) assessing updates to accounting policies, and (4) documenting new processes and controls. The Company expects to begin testing and sensitivity analysis on its initial modeling assumptions and results in the first quarter of 2019. ASU 2016-13 is effective for interim and annual periods in fiscal years beginning after December 15, 2019, with earlier adoption permitted. The Company plans to adopt this standard on January 1, 2020. Entities are required to use a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is adopted (modified-retrospective approach). A prospective transition approach is required for debt securities for which an other-than-temporary impairment had been recognized before the adoption date. The new standard will be significant to the policies, processes, and methodology used to determine credit losses; however, the Company has not yet determined the quantitative effect ASU 2016-13 will have on its consolidated financial position and results of operations. In January 2017, the FASB issued ASU 2017-04, "Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment," which intends to simplify goodwill impairment testing by eliminating the second step of the analysis under which the implied fair value of goodwill is determined as if the reporting unit were being acquired in a business combination. ASU 2017-04 instead requires entities to compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for any amount by which the carrying amount exceeds the reporting unit's fair value, to the extent that the loss recognized does not exceed the amount of goodwill allocated to that reporting unit. ASU 2017-04 must be applied prospectively and is effective for the Company on January 1, 2020. Early adoption is permitted. The Company does not expect ASU 2017-04 to have a material impact on its consolidated financial position or results of operations. In June 2018, the FASB issued ASU 2018-07, “Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting,” which simplifies the accounting for share-based payments granted to nonemployees for goods and services by aligning with the requirements for share-based payments granted to employees. The Company does not expect the adoption of ASU 2018-07 to have a material impact on its consolidated financial position or results of operations. In August 2018, the FASB issued ASU 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework - Changes to Disclosure Requirements for Fair Value Measurements,” which changes the fair value measurement disclosure requirements of ASC 820. ASU 2018-13 must be applied prospectively and is effective for the Company on January 1, 2020. Early adoption is permitted. The Company does not expect ASU 2018-13 to have a material impact on its consolidated financial position or results of operations. In August 2018, the FASB issued ASU 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract (a consensus of the FASB Emerging Issues Task Force)," which aligns the requirements for capitalizing implementation costs in a cloud computing arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The new guidance also prescribes the balance sheet, income statement, and cash flow classification of the capitalized implementation costs and related amortization expense, and requires additional quantitative and qualitative disclosures. ASU 2018-15 is effective for the Company on January 1, 2020 and the Company has the option to adopt the new standard either prospectively to eligible costs incurred on or after the date this guidance is first applied or retrospectively. Early adoption is permitted. The Company does not expect ASU 2018-15 to have a material impact on its consolidated financial position or results of operations.
Basis of Presentation	Basis of Presentation Our interim condensed consolidated financial statements are prepared in accordance with U.S. GAAP for interim financial information and pursuant to the requirements for reporting on Form 10-Q and Article 10 of Regulation S-X of the Securities Exchange Act of 1934. Accordingly, certain disclosures accompanying annual consolidated financial statements are omitted. In the opinion of management, all significant intercompany accounts and transactions have been eliminated and adjustments, consisting solely of normal recurring accruals and considered necessary for the fair presentation of financial statements for the interim periods, have been included. The current period's results of operations are not necessarily indicative of the results that ultimately may be achieved for the year. The interim condensed consolidated financial statements and notes thereto should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Form 10-K.
Use of Estimates	Use of Estimates We have made a number of estimates and assumptions related to the reporting of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting period to prepare these condensed consolidated financial statements in conformity with U.S. GAAP. Actual results could differ from those estimates. Material estimates subject to change in the near term include, among other items, the allowance for credit losses (the combination of the allowance for loan and lease losses and the reserve for unfunded loan commitments), the carrying value of intangible assets, the realization of deferred tax assets, and the fair value estimates of assets acquired and liabilities assumed in acquisitions. These estimates may be adjusted as more current information becomes available, and any adjustment may be significant. The allowance for loan and lease losses (“ALLL”) represents management’s estimate of probable credit losses inherent in the loan portfolio as of the balance sheet date. During the second quarter of 2018, the Company changed its ALLL methodology due to the growth and increased complexity of the loan portfolio. The new ALLL methodology included three primary changes: the quantitative component now employs a probability of default/loss given default ("PD/LGD") methodology; the loan segmentation groups our loan portfolio into 21 loan segments with similar risk characteristics (as opposed to 34 loan segments used under the previous methodology); and the historical range of loan performance history (often referred to as the look-back period) was lengthened by one year. The methodology for assessing individually impaired loans did not change under the new ALLL methodology. The ALLL methodology used to derive qualitative adjustments based on other internal or external factors was updated to align with the new PD/LGD methodology being applied to estimate the quantitative general allowance for unimpaired loans. As a result, the composition of the ALLL changed as the quantitative component increased and the qualitative component decreased as the new quantitative methodology now encompasses more information, such as the longer look-back period, that previously required a qualitative adjustment as part of determining the total ALLL estimate. These changes in the ALLL methodology did not result in material changes to management's overall estimate of the ALLL.
Reclassifications	Reclassifications Certain prior period amounts have been reclassified to conform to the current period’s presentation format. In our loan and allowance tables, we realigned our commercial loan portfolio classes and subclasses to better reflect and report our lending, especially in light of the fourth quarter of 2017 cash flow loan sale and the exiting of the origination operations related to general, technology, and healthcare cash flow loans. Prior to the realignment, our commercial portfolio classes were: (1) asset-based, (2) venture capital, (3) cash flow, and (4) equipment finance. After the realignment, our commercial portfolio classes are (1) asset-based (which includes equipment finance), (2) venture capital, and (3) other commercial (which includes retained cash flow). All of the loan and allowance tables, both current period and prior periods, reflect this realignment. Prior to January 1, 2018, our credit quality disclosures were only for Non-PCI loans and leases. As our gross PCI loan portfolio reduced to less than 0.4% of total loans as of the end of 2017, beginning in 2018 the credit quality disclosures reflect our entire loan and lease portfolio. Accordingly, for the credit quality tables in Note 6. Loans and Leases, amounts related to the 2018 periods are for total loans and leases, while amounts related to the 2017 periods are for Non-PCI loans and leases only.