Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 584.1	$ 557.3
Unrealized Gain	1.6	0.9
Unrealized Loss	(1.0)
Fair Value	584.7	558.2
Short-term investments \| Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	129.7	144.5
Unrealized Gain	0.1
Unrealized Loss	(0.1)
Fair Value	$ 129.7	$ 144.5
Short-term investments \| Commercial paper \| Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	1 year	1 year
Short-term investments \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 323.3	$ 270.5
Unrealized Gain	0.3	0.5
Unrealized Loss	(0.9)
Fair Value	$ 322.7	$ 271.0
Short-term investments \| Corporate debt securities \| Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	1 year	1 year
Short-term investments \| Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 131.1	$ 142.3
Unrealized Gain	1.2	0.4
Fair Value	$ 132.3	$ 142.7
Short-term investments \| Securities of government-sponsored entities \| Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	1 year	1 year
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 237.8	$ 299.3
Unrealized Gain	0.3	0.5
Unrealized Loss	(2.2)	(0.1)
Fair Value	235.9	299.7
Long-term investments \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	213.3	250.5
Unrealized Gain	0.2	0.5
Unrealized Loss	(2.2)	(0.1)
Fair Value	$ 211.3	$ 250.9
Long-term investments \| Corporate debt securities \| Minimum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	1 year	1 year
Long-term investments \| Corporate debt securities \| Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	2 years	2 years
Long-term investments \| Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 24.5	$ 48.8
Unrealized Gain	0.1
Fair Value	$ 24.6	$ 48.8
Long-term investments \| Securities of government-sponsored entities \| Minimum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	1 year	1 year
Long-term investments \| Securities of government-sponsored entities \| Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity	2 years	2 years

The term of contractual maturity for available for sale debt securities, in PnY format.

No definition available.

Name:	nbix_AvailableForSaleDebtSecuritiesContractualMaturity
Namespace Prefix:	nbix_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=nbix_SecuritiesOfGovernmentSponsoredEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=nbix_LongTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: