EMPLOYEE BENEFITS (Details) - 10K (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
EMPLOYEE BENEFITS
Annual expenses for incentive plans	$ 23.1rli_IncentivePlansBonusExpense	$ 23.2rli_IncentivePlansBonusExpense	$ 16.7rli_IncentivePlansBonusExpense
ESOP and 401(k) Plans
Employee Stock Ownership, 401(K) And Deferred Compensation Plan
Vesting period of profit sharing contributions	3 years
Expenses recognized	14.1	12.4	7.8
Employee Stock Ownership Plan (ESOP)
Employee Stock Ownership, 401(K) And Deferred Compensation Plan
Shares purchased under defined contribution plan	178,987	140,484	189,282
Average price of shares purchased (in dollars per share)	$ 44.43	$ 35.03	$ 35.01
Aggregate price of shares purchased	7.9	4.9	6.6
Shares held by the ESOP	3,482,617
Employee 401(k) Plans
Employee Stock Ownership, 401(K) And Deferred Compensation Plan
Employer contribution as a percent of eligible compensation	3.00%
Vesting percentage in basic and voluntary contributions	100.00%
Deferred compensation plan
Employee Stock Ownership, 401(K) And Deferred Compensation Plan
Shares purchased under defined contribution plan	9,920	13,922	14,968
Average price of shares purchased (in dollars per share)	$ 44.40	$ 38.33	$ 34.24
Aggregate price of shares purchased	0.4	0.5	0.5
Assets held under trust	$ 35.3

Name:	rli_DefinedContributionBenefitPlansLineItems
Namespace Prefix:	rli_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents the basic employer contribution to the defined contribution plan, as a percentage of eligible compensation.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanEmployerContribution
Namespace Prefix:	rli_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the shares purchased under the defined contribution plan.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanSharesPurchased
Namespace Prefix:	rli_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate market price of the shares purchased under the defined contribution plan.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanSharesPurchasedAggregateValue
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the average market price of the shares purchased under the defined contribution plan.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanSharesPurchasedAverageMarketPrice
Namespace Prefix:	rli_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the participant's vesting percentage in basic and voluntary contributions under the defined contribution plan.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanVestingPercentage
Namespace Prefix:	rli_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the period of time over which an employee's right to a profit-sharing contribution is no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

+ References

No definition available.

+ Details

Name:	rli_DefinedContributionPlanVestingPeriodOfProfitSharingContributions
Namespace Prefix:	rli_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the annual expense for bonuses awarded under incentive plans.

+ References

No definition available.

+ Details

Name:	rli_IncentivePlansBonusExpense
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

+ Details

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

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 40

-Section 50

-Paragraph 1

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929

+ Details

Name:	us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=rli_DeferredCompensationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=rli_EmployeeStockOwnershipPlanAndDefinedContributionPensionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=rli_EmployeeStockOwnershipPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: