Acquisition (Details) - USD ($) $ in Thousands	Oct. 20, 2017	Oct. 06, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Equipment leased to others under operating leases			$ 284,631	$ 229,905
Goodwill			$ 2,548,670	$ 2,173,949	$ 2,176,291	$ 1,720,479
Square 1 Financial, Inc. [Member]
Business Acquisition [Line Items]
Cash and due from banks		$ 24,867
Interest‑earning deposits in financial institutions		236,069
Securities available‑for‑sale		2,193,538
FHLB stock		2,787
Loans and leases		1,553,720
Premises and equipment		1,927
Goodwill		446,069
Core deposit and customer relationship intangibles		45,426
Other assets		106,757
Total assets acquired		4,611,160
Noninterest‑bearing deposits		2,549,000
Interest‑bearing deposits		1,240,635
Borrowings		0
Subordinated debentures		0
Accrued interest payable and other liabilities		24,092
Total liabilities assumed		3,813,727
Cash paid		0
PacWest common stock issued		797,433
Total consideration paid		$ 797,433
CU Bancorp [Member]
Business Acquisition [Line Items]
Cash and due from banks	$ 51,857
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
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
Total consideration paid	$ 670,571