Investment Securities - Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	$ 2	$ 6
Gross Unrealized Losses	956,507	811,136
Fair Value	4,487,172	4,843,487
Debt Securities, Available-for-sale, Amortized Cost, Total	5,443,677	5,654,617
Agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	535,855	442,996
Fair Value	2,000,900	2,242,042
Debt Securities, Available-for-sale, Amortized Cost, Total	2,536,755	2,685,038
Agency commercial MBS
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	67,572	61,886
Fair Value	470,894	487,606
Debt Securities, Available-for-sale, Amortized Cost, Total	538,466	549,492
Agency residential CMOs
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	73,326	60,111
Fair Value	417,979	457,063
Debt Securities, Available-for-sale, Amortized Cost, Total	491,305	517,174
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	2	0
Gross Unrealized Losses	107,030	101,075
Fair Value	664,202	670,070
Debt Securities, Available-for-sale, Amortized Cost, Total	771,230	771,145
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	59,826	60,398
Fair Value	336,531	339,326
Debt Securities, Available-for-sale, Amortized Cost, Total	396,357	399,724
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	3,188	6,898
Fair Value	105,980	102,261
Debt Securities, Available-for-sale, Amortized Cost, Total	109,168	109,159
Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	6
Gross Unrealized Losses	59,420	32,868
Fair Value	285,288	311,905
Debt Securities, Available-for-sale, Amortized Cost, Total	344,708	344,767
Private label commercial MBS
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1,728	2,076
Fair Value	21,653	26,827
Debt Securities, Available-for-sale, Amortized Cost, Total	23,381	28,903
SBA securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1,410	1,274
Fair Value	13,808	17,250
Debt Securities, Available-for-sale, Amortized Cost, Total	15,218	18,524
Private Label Collateralized Mortgage Obligations
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	46,824	40,399
Fair Value	148,946	166,724
Debt Securities, Available-for-sale, Amortized Cost, Total	195,770	207,123
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	328	1,155
Fair Value	20,991	22,413
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 21,319	$ 23,568

PACWEST BANCORP 10-Q