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Restrictions on Cash and Due from Banks (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Restrictions on Cash and Due from Banks
Amount of minimum average reserve balances required to be maintained	$ 0.0	$ 13.8
Balances at the Federal Reserve Bank and other financial institutions	$ 145.6	$ 109.4

X

- Definition

Represents the amount of minimum average reserve balances required to be maintained.

+ References

No definition available.

+ Details

Name:	alrs_AmountOfMinimumAverageReserveBalancesRequiredToBeMaintained
Namespace Prefix:	alrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

N/A

+ References

No definition available.

+ Details

Name:	alrs_RestrictionsOnCashAndDueFromBanksAbstract
Namespace Prefix:	alrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of cash subject to withdrawal restrictions, restricted deposits held as compensating balances, and cash and securities segregated in compliance with regulations (such as cash deposited in a special reserve account for the exclusive benefit of customers).

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndSecuritiesSegregatedUnderFederalAndOtherRegulations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant