For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Fair value measurements (Percentages of Total Commodity Sales Excluding Effects of Hedging Activities) (Details) - Customer Concentration Risk - Commodity Sales	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Coffeyville Resources LLC
Concentration Risk [Line Items]
Percentage of sales to significant purchaser			20.90%
Phillips 66 Company
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	21.40%	26.00%	14.60%
Sunoco, Inc.
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	15.10%	7.20%
Alta Mesa Resources, Inc.
Concentration Risk [Line Items]
Percentage of sales to significant purchaser		6.70%
Tom Stack LLC.
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	10.00%
Valero Energy Corporation
Concentration Risk [Line Items]
Percentage of sales to significant purchaser			13.30%