Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Provisions for Allowances and Accruals (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Allowance for Sales Return and Doubtful Accounts [Line Items]
Gross Auryxia product sales	$ 46,821	$ 26,029	$ 87,960	$ 43,983
Provision for product sales allowances and accruals	22,716	11,913	43,233	19,362
Net U.S. Auryxia product sales	$ 24,105	$ 14,116	$ 44,727	$ 24,621
Provision for product sales allowances and accruals, percentage	49.00%	46.00%	49.00%	44.00%
Trade allowances
Allowance for Sales Return and Doubtful Accounts [Line Items]
Provision for product sales allowances and accruals	$ 4,957	$ 2,463	$ 9,140	$ 4,228
Provision for product sales allowances and accruals, percentage	11.00%	9.00%	10.00%	9.00%
Rebates, chargebacks and discounts
Allowance for Sales Return and Doubtful Accounts [Line Items]
Provision for product sales allowances and accruals	$ 16,135	$ 8,784	$ 30,928	$ 14,114
Provision for product sales allowances and accruals, percentage	35.00%	34.00%	35.00%	32.00%
Product returns
Allowance for Sales Return and Doubtful Accounts [Line Items]
Provision for product sales allowances and accruals	$ 572	$ 346	$ 736	$ 278
Provision for product sales allowances and accruals, percentage	1.00%	2.00%	1.00%	1.00%
Other Incentives
Allowance for Sales Return and Doubtful Accounts [Line Items]
Provision for product sales allowances and accruals	$ 1,052	$ 320	$ 2,429	$ 742
Provision for product sales allowances and accruals, percentage	2.00%	1.00%	3.00%	2.00%

Allowance for Sales Return and Doubtful Accounts [Line Items]

No definition available.

Name:	kerx_AllowanceForSalesReturnAndDoubtfulAccountsLineItems
Namespace Prefix:	kerx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Sales Discounts Returns And Allowances Goods Percentage On Gross Sales

+ References

No definition available.

+ Details

Name:	kerx_SalesDiscountsReturnsAndAllowancesGoodsPercentageOnGrossSales
Namespace Prefix:	kerx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate of: (i) sales discounts given by the entity, including, but not limited to, early payments of accounts due, (ii) the total deduction from sales during the period arising from goods returned by customers (other than under warranty provisions), and (iii) price reductions (allowance, price protection agreements) given by the entity. Discounts, returns and allowances are a deduction from gross revenue in arriving at net revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 50
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68069731&loc=d3e63676-111659

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1)(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 15
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68051624&loc=d3e47468-111626

+ Details

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

- Definition

Aggregate revenue during the period from sale of goods in the normal course of business, before deducting returns, allowances and discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_SalesRevenueGoodsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

+ References

+ Details

Name:	us-gaap_SalesRevenueGoodsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=us-gaap_AllowanceForTradeReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=kerx_RebatesChargebacksAndDiscountsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=us-gaap_AllowanceForSalesReturnsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=kerx_OtherIncentivesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: