Deposits - Summary of Deposits (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deposits [Abstract]
Non-interest-bearing demand	$ 2,288,048	$ 2,199,983
Interest-bearing demand	854,935wal_InterestBearingDepositDemand	709,841wal_InterestBearingDepositDemand
Savings and money market	3,869,699us-gaap_DepositsSavingsDeposits	3,310,369us-gaap_DepositsSavingsDeposits
Certificate of deposits	1,339,238wal_TimeDeposits250000ormore	511,430wal_TimeDeposits250000ormore
Time Deposits, Less than $250,000	579,123wal_TimeDepositsLessthan250000	1,106,582wal_TimeDepositsLessthan250000
Total deposits	$ 8,931,043us-gaap_Deposits	$ 7,838,205us-gaap_Deposits

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.12)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 12

-Article 9

+ Details

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- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

+ Details

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- Definition

The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

+ References

+ Details

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- Definition

Interest Bearing Deposit Demand

+ References

No definition available.

+ Details

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- Definition

The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $250,000 or more.

+ References

No definition available.

+ Details

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- Definition

The aggregate amount of domestic or foreign time deposits (including certificates of deposits, individual retirement accounts and open accounts) that total to less than $250,000.

+ References

No definition available.

+ Details

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