Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 20, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Equipment leased to others under operating leases		$ 324,084	$ 292,677
Goodwill		2,548,670	2,548,670
PacWest common stock issued		$ 0	$ 0
CU Bancorp
Business Acquisition [Line Items]
Business Combinations, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash And Due From Banks	$ 51,857
Cash and due from banks	384,656
Interest‑earning deposits in financial institutions	332,799
Securities available‑for‑sale	446,980
FHLB stock	11,902
Loans and leases	2,075,890
Premises and equipment	2,981
Goodwill	374,721
Core deposit and customer relationship intangibles	57,500
Other assets	103,498
Total assets acquired	3,458,128
Noninterest‑bearing deposits	1,510,285
Interest‑bearing deposits	1,209,597
Business Combination Recognized Identifiable Assets And Liabilities Assumed, Total Deposits	2,719,882
Borrowings	22,879
Subordinated debentures	12,372
Accrued interest payable and other liabilities	32,424
Total liabilities assumed	2,787,557
Cash paid	224,338
PacWest common stock issued	446,233			$ 446,233
Total consideration paid	$ 670,571