Loans Receivable and Allowance for Credit Losses (Composition of Non-PCI and PCI Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	$ 28,975,718	$ 25,503,139
Allowance for loan losses	(287,128)	(260,520)
Loans held-for-investment, net	28,688,590	25,242,619
Commercial Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	22,210,001	19,917,365
Commercial Lending \| C&I
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	10,697,231	9,640,563
Commercial Lending \| CRE
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	8,936,897	8,016,109
Commercial Lending \| Residential loan \| Multifamily
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	1,916,176	1,585,939
Commercial Lending \| Construction and land
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	659,697	674,754
Consumer Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	6,765,717	5,585,774
Consumer Lending \| Residential loan \| Single-family
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	4,646,289	3,509,779
Consumer Lending \| HELOCs
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	1,782,924	1,760,776
Consumer Lending \| Other consumer
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	336,504	315,219
Non-PCI impaired loans
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	28,493,431	24,860,694
Allowance for loan losses	(287,070)	(260,402)
Loans held-for-investment, net	28,206,361	24,600,292
Unearned fees, premiums and discounts, net	(34,000)	1,200
Non-PCI impaired loans \| Commercial Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	21,859,099	19,432,958
Non-PCI impaired loans \| Commercial Lending \| C&I
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	10,685,436	9,602,176
Non-PCI impaired loans \| Commercial Lending \| CRE
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	8,659,209	7,667,661
Non-PCI impaired loans \| Commercial Lending \| Residential loan \| Multifamily
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	1,855,128	1,490,285
Non-PCI impaired loans \| Commercial Lending \| Construction and land
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	659,326	672,836
Non-PCI impaired loans \| Consumer Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	6,634,332	5,427,736
Non-PCI impaired loans \| Consumer Lending \| Residential loan \| Single-family
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	4,528,911	3,370,669
Non-PCI impaired loans \| Consumer Lending \| HELOCs
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	1,768,917	1,741,852
Non-PCI impaired loans \| Consumer Lending \| Other consumer
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	336,504	315,215
PCI Loans
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	482,287	642,445
Allowance for loan losses	(58)	(118)
Loans held-for-investment, net	482,229	642,327
Discount related to ASC 310-30	35,300	49,400
PCI Loans \| Commercial Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	350,902	484,407
PCI Loans \| Commercial Lending \| C&I
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	11,795	38,387
PCI Loans \| Commercial Lending \| CRE
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	277,688	348,448
PCI Loans \| Commercial Lending \| Residential loan \| Multifamily
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	61,048	95,654
PCI Loans \| Commercial Lending \| Construction and land
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	371	1,918
PCI Loans \| Consumer Lending
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	131,385	158,038
PCI Loans \| Consumer Lending \| Residential loan \| Single-family
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	117,378	139,110
PCI Loans \| Consumer Lending \| HELOCs
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	14,007	18,924
PCI Loans \| Consumer Lending \| Other consumer
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Total loans held-for-investment	$ 0	$ 4

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A valuation allowance for financing receivables that are expected to be uncollectible.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
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Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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