Related-Party Transactions, Summary of Affiliate Transactions (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended																12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Related Party Transaction
Revenues			$ 623	$ 642		$ 569		$ 546		$ 1,114		$ 1,094		$ 614		$ 427		$ 2,380	[2]	$ 3,249	[2]	$ 1,669	[2]
Operating and maintenance expenses	[3]																	272		251		308
General and administrative expenses																		98		101		80
Imbalance settlement gains																		(16)		(12)		(7)
Reimbursements from Andeavor																		15		16		17
Quarterly cash distributions, including IDRs																		(493)		(336)		(245)
Subsequent Event
Related Party Transaction
Quarterly cash distributions, including IDRs		$ (146)
Distribution Made to Limited Partner, Declaration Date		Jan. 25, 2019
Date of distribution		Feb. 14, 2019
Distribution Made to Limited Partner, Date of Record		Feb. 05, 2019
Secondment and Logistics Services Agreement | Subsequent Event
Related Party Transaction
Length Of Time For Continuing Minimum Payments		60 days
Term Of Agreement		10 years
Secondment and Logistics Services Agreement | Partner to Andeavor [Member]
Related Party Transaction
Long-term Purchase Commitment, Amount																		8		8		5
Secondment and Logistics Services Agreement | Andeavor to Partner [Member]
Related Party Transaction
Long-term Purchase Commitment, Amount																		77		29		18
Affiliated Entity [Member]
Related Party Transaction
Revenues																		$ 1,589		$ 1,431		$ 847
Omnibus Agreement
Related Party Transaction
Years To Identify Conditions For Indemnification																		5 years
Common | Subsequent Event
Related Party Transaction
Cash distributions paid per unit		$ 1.03

[1] Adjusted to include the historical results of the Predecessors. See Notes 1 and 2 for further discussion.

[2] All periods include the historical results of the Predecessors. See Notes 1 and 2 for further discussion.

[3] Includes net imbalance settlement gains of $12 million and $7 million in the years ended December 31, 2017 and 2016, respectively. Due to the adoption of ASC 606 effective January 1, 2018, imbalance settlement gains of $16 million in the year ended December 31, 2018 were included in revenues. Includes reimbursements primarily related to pressure testing and repairs and maintenance costs pursuant to the Amended Omnibus Agreement and the Carson Assets Indemnity Agreement of $15 million, $16 million and $17 million in the years ended December 31, 2018, 2017 and 2016, respectively.