Acquisition (Details) - USD ($) $ in Thousands				12 Months Ended
	Oct. 06, 2015	Apr. 07, 2014	May. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Equipment leased to others under operating leases				$ 197,452	$ 122,506
Goodwill				2,176,291	1,720,479	$ 208,743	$ 79,866
Square 1 Financial, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable				$ 797,433
Cash and due from banks	$ 24,867
Interest‑earning deposits in financial institutions	236,069
Investment securities available‑for‑sale	2,193,538
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
FDIC loss sharing asset	0
Income tax assets	0
Goodwill	447,911
Core deposit and customer relationship intangibles	45,426
Other assets	106,757
Total assets acquired	4,613,002
Noninterest‑bearing deposits	2,549,000
Interest‑bearing deposits	1,240,635
Other borrowings	0
Subordinated debentures	0
Discontinued operations	0
Accrued interest payable and other liabilities	25,934
Total liabilities assumed	3,815,569
Total consideration paid	797,433
Cash paid	0
Cancellation of FCAL common stock owned by PacWest (at acquisition date fair value)	$ 0
Capitalsource, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable					$ 2,594,070
Cash and due from banks		$ 768,553
Interest‑earning deposits in financial institutions		60,612
Investment 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
FDIC loss sharing asset		0
Income tax assets		304,856
Goodwill		1,526,282
Core deposit and customer relationship intangibles		6,720
Other assets		582,985
Total assets acquired		10,735,352
Noninterest‑bearing deposits		4,631
Interest‑bearing deposits		6,236,419
Other borrowings		992,109
Subordinated debentures		300,918
Discontinued operations		0
Accrued interest payable and other liabilities		124,087
Total liabilities assumed		7,658,164
Total consideration paid		3,077,188
Cash paid		483,118
Cancellation of FCAL common stock owned by PacWest (at acquisition date fair value)		$ 0
First California Financial Group, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable						$ 242,268
Cash and due from banks			$ 6,124
Interest‑earning deposits in financial institutions			266,889
Investment securities available‑for‑sale			4,444
FHLB stock			9,518
Loans and leases			1,049,613
Equipment leased to others under operating leases			0
Premises and equipment			15,322
Foreclosed assets			13,772
FDIC loss sharing asset			17,241
Income tax assets			33,360
Goodwill			129,070
Core deposit and customer relationship intangibles			7,927
Other assets			27,576
Total assets acquired			1,580,856
Noninterest‑bearing deposits			361,166
Interest‑bearing deposits			739,713
Other borrowings			0
Subordinated debentures			24,061
Discontinued operations			184,619
Accrued interest payable and other liabilities			19,729
Total liabilities assumed			1,329,288
Total consideration paid			251,568
Cash paid			0
Cancellation of FCAL common stock owned by PacWest (at acquisition date fair value)			$ 9,300