Cash Equivalents and Marketable Debt Securities - Schedule of Cash Equivalents and Marketable Debt Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 126,238	$ 137,759
Unrealized Gains	1	5
Unrealized Losses	(555)	(386)
Fair Value	125,684	137,378
Current Assets
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	110,837	107,890
Unrealized Gains	1	5
Unrealized Losses	(245)	(117)
Fair Value	110,593	107,778
Current Assets \| Government Sponsored Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,508	19,261
Unrealized Gains	0	0
Unrealized Losses	(15)	(28)
Fair Value	12,493	19,233
Current Assets \| Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	86,952	82,188
Unrealized Gains	1	5
Unrealized Losses	(222)	(84)
Fair Value	86,731	82,109
Current Assets \| Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	5,441
Unrealized Gains	0
Unrealized Losses	(3)
Fair Value	5,438
Current Assets \| Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	5,936	6,441
Unrealized Gains	0	0
Unrealized Losses	(5)	(5)
Fair Value	5,931	6,436
Noncurrent Assets
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,401	29,869
Unrealized Gains	0	0
Unrealized Losses	(310)	(269)
Fair Value	15,091	29,600
Noncurrent Assets \| Government Sponsored Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,759	2,760
Unrealized Gains	0	0
Unrealized Losses	(62)	(43)
Fair Value	2,697	2,717
Noncurrent Assets \| Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,642	27,109
Unrealized Gains	0	0
Unrealized Losses	(248)	(226)
Fair Value	$ 12,394	$ 26,883

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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 investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
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_BalanceSheetLocationAxis=nk_CurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=nk_NoncurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: