Acquisition (Details) - USD ($) $ in Thousands	Oct. 06, 2015	Apr. 07, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Equipment leased to others under operating leases			$ 229,905	$ 197,452
Goodwill			$ 2,173,949	$ 2,176,291	$ 1,720,479	$ 208,743
Square 1 Financial, Inc. [Member]
Business Acquisition [Line Items]
Cash and due from banks	$ 24,867
Interest‑earning deposits in financial institutions	236,069
FHLB stock	2,787
Loans and leases	1,553,720
Equipment leased to others under operating leases	0
Premises and equipment	1,927
Foreclosed assets	0
Goodwill	446,069
Core deposit and customer relationship intangibles	45,426
Income tax assets	0
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
Capitalsource, Inc. [Member]
Business Acquisition [Line Items]
Cash and due from banks		$ 768,553
Interest‑earning deposits in financial institutions		60,612
Securities available‑for‑sale		382,797
FHLB stock		46,060
Loans and leases		6,877,427
Equipment leased to others under operating leases		160,015
Premises and equipment		12,663
Foreclosed assets		6,382
Goodwill		1,526,282
Core deposit and customer relationship intangibles		6,720
Income tax assets		304,856
Other assets		582,985
Total assets acquired		10,735,352
Noninterest‑bearing deposits		4,631
Interest‑bearing deposits		6,236,419
Borrowings		992,109
Subordinated debentures		300,918
Accrued interest payable and other liabilities		124,087
Total liabilities assumed		7,658,164
Cash paid		483,118
PacWest common stock issued		2,594,070
Total consideration paid		$ 3,077,188