Westlake Chemical Partners (WLKP) 10-Q 2 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 25, 2019
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	WLKP
Entity Registrant Name	WESTLAKE CHEMICAL PARTNERS LP
Entity Central Index Key	0001604665
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Units, Units Outstanding		35,188,189

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 84,615	$ 19,744
Receivable under the Investment Management Agreement—Westlake Chemical Corporation (Westlake)	276,501	148,956
Accounts receivable, net—Westlake	47,626	57,280
Accounts receivable, net—third parties	15,495	16,404
Inventories	4,171	4,388
Prepaid expenses and other current assets	251	370
Total current assets	428,659	247,142
Property, plant and equipment, net	1,138,434	1,148,265
Goodwill	5,814	5,814
Deferred charges and other assets, net	58,366	60,904
Total assets	1,631,273	1,462,125
Current liabilities
Accounts payable—Westlake	19,344	27,477
Accounts payable—third parties	7,481	5,045
Accrued and other liabilities	20,433	16,250
Total current liabilities	47,258	48,772
Long-term debt payable to Westlake	601,119	477,608
Deferred income taxes	1,677	1,664
Operating lease liabilities	1,272	0
Total liabilities	651,326	528,044
Commitments and contingencies (Note 12)
EQUITY
Total Westlake Chemical Partners LP partners' capital	279,742	215,810
Noncontrolling interest in Westlake Chemical OpCo LP (OpCo)	700,205	718,271
Total equity	979,947	934,081
Total liabilities and equity	1,631,273	1,462,125
Limited Partner [Member] | Common units [Member] | Publicly and Privately Held [Member]
EQUITY
Limited Partners' Capital Account	473,119	409,608
Total equity	473,119	409,608
Limited Partner [Member] | Common units [Member] | Westlake [Member]
EQUITY
Limited Partners' Capital Account	49,195	48,774
Total equity	49,195	48,774
General Partner [Member] | Westlake [Member]
EQUITY
General partner—Westlake	(242,572)	(242,572)
Total equity	$ (242,572)	$ (242,572)

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - Common units [Member] - shares	Mar. 31, 2019	Dec. 31, 2018
Publicly and Privately Held [Member]
Units issued	21,065,959	18,125,141
Units outstanding	21,065,959	18,125,141
Westlake [Member]
Units issued	14,122,230	14,122,230
Units outstanding	14,122,230	14,122,230

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net sales—Westlake	$ 257,040	$ 235,031
Total net sales	299,086	284,272
Cost of sales	208,432	191,767
Gross profit	90,654	92,505
Selling, general and administrative expenses	6,973	7,133
Income from operations	83,681	85,372
Other income (expense)
Interest expense—Westlake	(5,900)	(4,866)
Other income, net	815	491
Income before income taxes	78,596	80,997
Income tax provision	200	283
Net income	78,396	80,714
Less: Net income attributable to noncontrolling interest in OpCo	63,441	68,419
Net income attributable to Westlake Chemical Partners LP	$ 14,955	$ 12,295
Weighted average limited partner units outstanding (basic and diluted)
Weighted average limited partner units outstanding (basic and diluted) (in shares)	32,345,398	32,236,449
Common units [Member]
Other income (expense)
Net income attributable to Westlake Chemical Partners LP	$ 14,955	$ 11,562
Net income per limited partner unit attributable to Westlake Chemical Partners LP (basic and diluted)
Income per limited partner unit (usd per share)	$ 0.46	$ 0.36
Weighted average limited partner units outstanding (basic and diluted)
Weighted average limited partner units outstanding (basic and diluted) (in shares)	32,345,398	32,236,449
Common units [Member] | Publicly and Privately Held [Member]
Weighted average limited partner units outstanding (basic and diluted)
Weighted average limited partner units outstanding (basic and diluted) (in shares)	18,223,168	18,115,036
Common units [Member] | Westlake [Member]
Weighted average limited partner units outstanding (basic and diluted)
Weighted average limited partner units outstanding (basic and diluted) (in shares)	14,122,230	14,122,230
Product and Service, Other [Member]
Total net sales	$ 42,046	$ 49,241

Consolidated Statements of Chan

Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest in OpCo [Member]	Westlake [Member]General Partner [Member]	Common units [Member]Publicly and Privately Held [Member]Limited Partner [Member]	Common units [Member]Westlake [Member]Limited Partner [Member]
Equity, beginning balance at Dec. 31, 2017	$ 998,749	$ 279	$ 778,935	$ (241,958)	$ 411,228	$ 50,265
Net income	80,714	0	68,419	733	6,498	5,064
Net effect of cash flow hedge	19	19	0	0	0	0
Units issued for vested phantom units	60	0	0	0	60	0
Proceeds from private placement of common units	0
Quarterly distributions to unitholders	(13,071)	0	0	(614)	(7,000)	(5,457)
Quarterly distribution to noncontrolling interest retained in OpCo by Westlake	(91,148)	0	(91,148)	0	0	0
Equity, ending balance at Mar. 31, 2018	975,323	298	756,206	(241,839)	410,786	49,872
Equity, beginning balance at Dec. 31, 2018	934,081	0	718,271	(242,572)	409,608	48,774
Net income	78,396	0	63,441	0	8,422	6,533
Proceeds from private placement of common units	62,934	0	0	0	62,934	0
Quarterly distributions to unitholders	(13,957)	0	0	0	(7,845)	(6,112)
Quarterly distribution to noncontrolling interest retained in OpCo by Westlake	(81,507)	0	(81,507)	0	0	0
Equity, ending balance at Mar. 31, 2019	$ 979,947	$ 0	$ 700,205	$ (242,572)	$ 473,119	$ 49,195

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income	$ 78,396	$ 80,714
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization	26,844	27,698
Loss from disposition of property, plant and equipment	458	567
Other losses (gains), net	(180)	357
Changes in operating assets and liabilities
Accounts receivable—third parties	1,102	(1,239)
Net accounts receivable—Westlake	1,479	737
Inventories	(587)	419
Prepaid expenses and other current assets	119	153
Accounts payable	2,497	(2,835)
Accrued and other liabilities	3,472	(487)
Other, net	(101)	138
Cash flows from investing activities	113,499	106,222
Cash flows from investing activities
Additions to property, plant and equipment	(12,144)	(9,679)
Maturities of investments with Westlake under the Investment Management Agreement	80,000	112,000
Investments with Westlake under the Investment Management Agreement	(207,512)	(110,000)
Other	46	0
Net cash used for investing activities	(139,610)	(7,679)
Cash flows from financing activities
Proceeds from private placement of common units	62,934	0
Proceeds from debt payable to Westlake	123,512	3,648
Quarterly distributions to noncontrolling interest retained in OpCo by Westlake	(81,507)	(91,148)
Quarterly distributions to unitholders	(13,957)	(13,071)
Net cash provided by (used for) financing activities	90,982	(100,571)
Net increase (decrease) in cash and cash equivalents	64,871	(2,028)
Cash and cash equivalents at beginning of period	19,744	27,008
Cash and cash equivalents at end of period	$ 84,615	$ 24,980

Description of Business and Bas

Description of Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business and Basis of Presentation	Description of Business and Basis of Presentation Description of Business Westlake Chemical Partners LP (the "Partnership") is a Delaware limited partnership formed in March 2014 to operate, acquire and develop ethylene production facilities and related assets. On August 4, 2014, the Partnership completed its initial public offering (the "IPO") of 12,937,500 common units representing limited partner interests. On September 29, 2017, the Partnership completed a secondary offering of 5,175,000 common units at a price of $22.00 per unit. On March 29, 2019, the Partnership completed the issuance and sale of 2,940,818 common units at a price of $21.40 per unit through a private placement. Proceeds to Westlake Partners from the sale of the units were approximately $62,934 . In connection with the IPO, the Partnership acquired a 10.6% interest in Westlake Chemical OpCo LP ("OpCo") and a 100% interest in Westlake Chemical OpCo GP LLC ("OpCo GP"), which is the general partner of OpCo. On April 29, 2015, the Partnership purchased an additional 2.7% newly-issued limited partner interest in OpCo for approximately $135,341 , resulting in an aggregate 13.3% limited partner interest in OpCo, effective April 1, 2015. On September 29, 2017, the Partnership purchased an additional 5.0% newly-issued limited partner interest in OpCo for approximately $229,207 , resulting in an aggregate 18.3% limited partner interest in OpCo, effective as of July 1, 2017. On March 29, 2019, the Partnership purchased an additional 4.5% newly-issued limited partner interest in OpCo for approximately $201,445 , resulting in an aggregate 22.8% limited partner interest in OpCo, effective January 1, 2019. The remaining 77.2% limited partner interest in OpCo is owned by Westlake Chemical Corporation. OpCo owns three ethylene production facilities and a common carrier ethylene pipeline. Basis of Presentation The accompanying unaudited consolidated interim financial statements were prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the "SEC") for interim periods. Accordingly, certain information and footnotes required for complete financial statements under generally accepted accounting principles in the United States ("U.S. GAAP") have not been included. These interim consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto of the Partnership included in the annual report on Form 10-K for the fiscal year ended December 31, 2018 (the " 2018 Form 10-K"), filed with the SEC on March 1, 2019 . These financial statements have been prepared in conformity with the accounting principles and practices as disclosed in the notes to the consolidated financial statements of the Partnership for the fiscal year ended December 31, 2018 with the exceptions of those accounting standards adopted in 2019 as discussed in Note 1 . References to "Westlake" refer collectively to Westlake Chemical Corporation and its subsidiaries, other than the Partnership, OpCo and OpCo GP. The Partnership holds a 22.8% limited partner interest and the entire non-economic general partner interest in OpCo. The remaining 77.2% limited partner interest in OpCo is owned by Westlake, which has no rights to direct the activities that most significantly impact the economic performance of OpCo. As a result of the fact that substantially all of OpCo's activities are conducted on behalf of Westlake, and the fact that OpCo exhibits disproportionality of voting rights to economic interest, OpCo was deemed to be a variable interest entity. The Partnership, through its ownership of OpCo's general partner, has the power to direct the activities that most significantly impact the economic performance of OpCo, and it also has the obligation or right to absorb losses or receive benefits from OpCo that could potentially be significant to OpCo. As such, the Partnership was determined to be OpCo's primary beneficiary and therefore consolidates OpCo's results of operations and financial position. Westlake's retained interest of 77.2% is recorded as noncontrolling interest in the Partnership's consolidated financial statements. In the opinion of the Partnership's management, the accompanying unaudited consolidated interim financial statements reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair statement of the Partnership's financial position as of March 31, 2019 , its results of operations for the three months ended March 31, 2019 and 2018 and the changes in its cash position for the three months ended March 31, 2019 and 2018 . Results of operations and changes in cash position for the interim periods presented are not necessarily indicative of the results that will be realized for the fiscal year ending December 31, 2019 or any other interim period. The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities. Actual results could differ materially from those estimates. Recent Accounting Pronouncements Credit Losses (ASU No. 2016-13) In June 2016, the FASB issued an accounting standards update providing new guidance for the accounting for credit losses on loans and other financial instruments. The new guidance introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments. The standard also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. The accounting standard will be effective for reporting periods beginning after December 15, 2019 and is not expected to have a material impact on the Partnership's consolidated financial position, results of operations and cash flows. Fair Value Measurement (ASU No. 2018-13) In August 2018, the FASB issued an accounting standards update to modify the disclosure requirements on fair value measurements. The amendments are effective for reporting periods beginning after December 15, 2019. An entity is permitted to early adopt any removed or modified disclosures and delay adoption of the additional disclosures until the effective date. Most amendments should be applied retrospectively but certain amendments should be applied prospectively. The Partnership is in the process of evaluating the impact that the new accounting guidance will have on the Partnership's consolidated financial position, results of operations and cash flows. Recently Adopted Accounting Standards Leases (ASU No. 2016-02) In February 2016, the FASB issued an accounting standards update on lease accounting that supersedes the previously issued lease guidance. The new standard requires lessees to recognize assets and liabilities for all long-term operating leases. An asset is recognized for the right to use an underlying leased asset and a liability is recognized for the obligation to make payments over the lease term. The standard also requires expanded lease disclosures. The standard requires a modified retrospective adoption approach and allows for the election of certain transition expedients. The Partnership adopted the standard effective January 1, 2019 using the optional transition method which allows entities to recognize a cumulative adjustment to the opening balance sheet in the period of adoption. The Partnership elected the package of optional transition expedients and was not required to reassess (1) whether any existing contracts are or contain leases, (2) classification of existing leases as operating or capital or (3) whether initial direct costs for existing leases qualify for capitalization under the new accounting standard. The Partnership did not elect the use of hindsight to determine the lease term when considering lease renewal or termination options. Adoption of the new standard did not have a material impact on the Partnership's consolidated financial position, results of operations and cash flows.

Accounts Receivable - Third Par

Accounts Receivable - Third Parties	3 Months Ended
	Mar. 31, 2019
Accounts Receivable, Net [Abstract]
Accounts Receivable - Third Parties	Accounts Receivable—Third Parties Accounts receivable—third parties consist of the following: March 31, December 31, Trade customers $ 14,455 $ 17,325 Allowance for doubtful accounts (728 ) (921 ) 13,727 16,404 Other 1,768 — Accounts receivable, net—third parties $ 15,495 $ 16,404

Inventories

Inventories	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: March 31, December 31, Finished products $ 3,741 $ 3,876 Feedstock, additives and chemicals 430 512 Inventories $ 4,171 $ 4,388

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Depreciation expense on property, plant and equipment of $22,214 and $22,157 is included in cost of sales in the consolidated statements of operations for the three months ended March 31, 2019 and 2018 , respectively.

Deferred Charges and Other Asse

Deferred Charges and Other Assets	3 Months Ended
	Mar. 31, 2019
Other Assets, Noncurrent [Abstract]
Deferred Charges and Other Assets	Deferred Charges and Other Assets Amortization expense on other assets of $4,630 and $5,541 is included in costs of sales in the consolidated statements of operations for the three months ended March 31, 2019 and 2018 , respectively.

Distributions and Net Income Pe

Distributions and Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Distributions and Net Income Per Limited Partner Unit	Distributions and Net Income Per Limited Partner Unit On April 30, 2019 , the board of directors of Westlake Chemical Partners GP LLC ("Westlake GP"), the Partnership's general partner, declared a quarterly cash distribution for the three months ended March 31, 2019 of $0.4452 per unit. This distribution is payable on May 28, 2019 to the unitholders of record as of May 13, 2019 . The distributions are declared subsequent to quarter end; therefore, the table below represents total distributions declared from earnings of the related periods pertaining to such distributions. Three Months Ended March 31, 2019 2018 Net income attributable to the Partnership $ 14,955 $ 12,295 Less: Limited partners' distributions declared on common units 15,666 12,814 Distributions declared with respect to the incentive distribution rights — 733 Net income in excess of distribution (distribution in excess of net income) $ (711 ) $ (1,252 ) Net income per unit applicable to common limited partner units is computed by dividing the respective limited partners' interest in net income by the weighted-average number of common units outstanding for the period. Because the Partnership has more than one class of participating securities, it uses the two-class method when calculating the net income per unit applicable to limited partners. The classes of participating securities include common units and incentive distribution rights. Net income attributable to the Partnership is allocated to the unitholders in accordance with their respective ownership percentages in preparation of the consolidated statement of equity. However, when distributions related to the incentive distribution rights are made, net income equal to the amount of those distributions is first allocated to the general partner before the remaining net income is allocated to the unitholders based on their respective ownership percentages. Basic and diluted net income per unit is the same because the Partnership does not have any potentially dilutive units outstanding for the periods presented. Three Months Ended March 31, 2019 Limited Partners' Common Units Incentive Distribution Rights Total Net income attributable to the Partnership: Distribution declared $ 15,666 $ — $ 15,666 Distribution in excess of net income (711 ) — (711 ) Net income $ 14,955 $ — $ 14,955 Weighted average units outstanding: Basic and diluted 32,345,398 32,345,398 Net income per limited partner unit: Basic and diluted $ 0.46 Three Months Ended March 31, 2018 Limited Partners' Common Units Incentive Distribution Rights Total Net income attributable to the Partnership: Distribution declared $ 12,814 $ 733 $ 13,547 Distribution in excess of net income (1,252 ) — (1,252 ) Net income $ 11,562 $ 733 $ 12,295 Weighted average units outstanding: Basic and diluted 32,236,449 32,236,449 Net income per limited partner unit: Basic and diluted $ 0.36 The amended Partnership Agreement provides that the Partnership will distribute cash each quarter to all the unitholders, pro rata, until each unit has received a distribution of $1.2938 . If cash distributions to the Partnership's unitholders exceed $1.2938 per unit in any quarter, the Partnership's unitholders and Westlake, as the holder of the Partnership's incentive distribution rights, will receive distributions according to the following percentage allocations: Marginal Percentage Interest in Distributions Total Quarterly Distribution Per Unit Unitholders IDR Holders Above $1.2938 up to $1.4063 85.0 % 15.0 % Above $1.4063 up to $1.6875 75.0 % 25.0 % Above $1.6875 50.0 % 50.0 % The Partnership's distribution for the three months ended March 31, 2019 did not exceed the $1.2938 per unit threshold, and, as a result, no distribution was made with respect to the Partnership's incentive distribution rights to Westlake, as the holder of the Partnership's incentive distribution rights. Distribution Per Common Unit Distributions per common unit for the three months ended March 31, 2019 and 2018 were as follows: Three Months Ended March 31, 2019 2018 Distributions per common unit $ 0.4328 $ 0.3864

Partners' Equity

Partners' Equity	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Partners' Equity	7. Partners' Equity On October 4, 2018, the Partnership and Westlake GP, the general partner of the Partnership, entered into an Equity Distribution Agreement with UBS Securities LLC, Barclays Capital Inc., Citigroup Global Markets Inc., Deutsche Bank Securities Inc., RBC Capital Markets, LLC, Merrill Lynch, Pierce, Fenner & Smith Incorporated and Wells Fargo Securities, LLC to offer and sell the Partnership's common units, from time to time, up to an aggregate offering amount of $50,000 . No common units had been issued under this program as of March 31, 2019 . On March 29, 2019, the Partnership completed the issuance and sale of 2,940,818 common units at a price of $21.40 per unit through a private placement. Proceeds to the Partnership from the sale of the units were approximately $62,934 . TTWF LP, Westlake's principal stockholder and a related party, acquired 1,401,869 common units out of 2,940,818 common units issued in the private placement.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Partnership and OpCo regularly enter into related party transactions with Westlake. See below for a description of transactions with related parties. Sales to Related Parties OpCo sells ethylene to Westlake under the Ethylene Sales Agreement. Additionally, the Partnership and OpCo from time to time provide other services or products for which each charges Westlake a fee. Sales to related parties were as follows: Three Months Ended March 31, 2019 2018 Net sales—Westlake $ 257,040 $ 235,031 Cost of Sales from Related Parties Charges for goods and services purchased by the Partnership and OpCo from Westlake and included in cost of sales relate primarily to feedstock purchased under the Feedstock Supply Agreement and services provided under the Services and Secondment Agreement. Charges from related parties in cost of sales were as follows: Three Months Ended March 31, 2019 2018 Feedstock purchased from Westlake and included in cost of sales $ 122,155 $ 105,415 Other charges from Westlake and included in cost of sales 27,572 28,406 Total $ 149,727 $ 133,821 Services from Related Parties Included in Selling, General and Administrative Expenses Charges for services purchased by the Partnership from Westlake and included in selling, general and administrative expenses primarily relate to services Westlake performs on behalf of the Partnership under the Omnibus Agreement, including the Partnership's finance, legal, information technology, human resources, communication, ethics and compliance and other administrative functions. Charges from related parties included within selling, general and administrative expenses were as follows: Three Months Ended March 31, 2019 2018 Services received from Westlake and included in selling, general and administrative expenses $ 6,603 $ 6,251 Goods and Services from Related Parties Capitalized as Assets Charges for goods and services purchased by the Partnership and OpCo from Westlake which were capitalized as assets relate primarily to the services of Westlake employees under the Services and Secondment Agreement. Charges from related parties for goods and services capitalized as assets were as follows: Three Months Ended March 31, 2019 2018 Goods and services purchased from Westlake and capitalized as assets $ 643 $ 351 Receivable under the Investment Management Agreement On August 1, 2017, the Partnership, OpCo and Westlake executed an investment management agreement (the "Investment Management Agreement") that authorized Westlake to invest the Partnership's and OpCo's excess cash with Westlake for a term of up to a maximum of nine months. Per the terms of the Investment Management Agreement, the Partnership earns a market return plus five basis points and Westlake provides daily availability of the invested cash to meet any liquidity needs of the Partnership or OpCo. Accrued interest of $34 and $496 was included in the receivable under the Investment Management Agreement balance at March 31, 2019 and December 31, 2018 , respectively. Total interest earned related to the Investment Management Agreement was $818 and $540 for the three months ended March 31, 2019 and 2018 , respectively. The Partnership's receivable under the Investment Management Agreement was as follows: March 31, December 31, Receivable under the Investment Management Agreement $ 276,501 $ 148,956 Accounts Receivables The Partnership's accounts receivable from Westlake result primarily from ethylene sales to Westlake under the Ethylene Sales Agreement. The Partnership's accounts receivable from Westlake were as follows: March 31, December 31, Accounts receivable—Westlake $ 47,626 $ 57,280 Accounts Payable to Related Parties The Partnership's accounts payable to Westlake result primarily from feedstock purchases under the Feedstock Supply Agreement and services provided under the Services and Secondment Agreement and the Omnibus Agreement. The related party accounts payable balances were as follows: March 31, December 31, Accounts payable—Westlake $ 19,344 $ 27,477 Related Party Leases OpCo is obligated to Westlake under various rail cars leases. Operating lease rentals paid to Westlake for such leases were $514 and $318 for the three months ended March 31, 2019 and 2018 , respectively, and reflected in other charges from Westlake that are included in cost of sales. OpCo has two site lease agreements with Westlake, each of which has a term of 50 years. Pursuant to the site lease agreements, OpCo pays Westlake one dollar per site per year. Debt Payable to Related Parties See Note 9 for a description of related party debt payable balances. Interest on related party debt payable balances for the three months ended March 31, 2019 and 2018 was $5,900 and $4,866 , respectively. Interest on related party debt payable is presented as interest expense—Westlake in the consolidated statements of operations. Interest capitalized as a component of property, plant and equipment on related party debt was zero and $15 for the three months ended March 31, 2019 and 2018 , respectively. At March 31, 2019 and December 31, 2018 , accrued interest on related party debt was $5,852 and $5,448 , respectively, and is reflected as a component of accrued liabilities in the consolidated balance sheets. Debt payable to related parties was as follows: March 31, December 31, Long-term debt payable to Westlake $ 601,119 $ 477,608 Major Customer and Concentration of Credit Risk During the three months ended March 31, 2019 and 2018 , Westlake accounted for approximately 85.9% and 82.7% , respectively, of the Partnership's net sales. Other See Note 7 above for an additional related party transaction.

Long-term Debt Payable to Westl

Long-term Debt Payable to Westlake	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Long-term Debt Payable to Westlake	Long-term Debt Payable to Westlake Long-term debt payable to Westlake consists of the following: March 31, December 31, OpCo Revolver (variable interest rate of LIBOR plus 2.0%, scheduled maturity of September 25, 2023) $ 224,064 $ 224,064 MLP Revolver (variable interest rate of LIBOR plus 2.0%, scheduled maturity of April 29, 2021) 377,055 253,544 $ 601,119 $ 477,608 On March 29, 2019, the Partnership borrowed $123,512 under the MLP Revolver to partially fund the purchase of the additional 4.5% interest in OpCo. The weighted average interest rate on all long-term debt was 4.8% and 4.4% , respectively, at March 31, 2019 and December 31, 2018 . As of March 31, 2019 , the Partnership was in compliance with all of the covenants under the OpCo Revolver and the MLP Revolver.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Partnership reports certain assets and liabilities at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). Under the accounting guidance for fair value measurements, inputs used to measure fair value are classified in one of three levels: Level 1: Quoted market prices in active markets for identical assets or liabilities. Level 2: Observable market-based inputs or unobservable inputs that are corroborated by market data. Level 3: Unobservable inputs that are not corroborated by market data. The Partnership has financial assets and liabilities subject to fair value measures. These financial assets and liabilities include accounts receivable, net, accounts payable and long-term debt payable to Westlake, all of which are recorded at carrying value. The amounts reported in the consolidated balance sheets for accounts receivable, net and accounts payable approximate their fair value due to the short maturities of these instruments. The carrying and fair values of the Partnership's long-term debt at March 31, 2019 and December 31, 2018 are summarized in the table below. The Partnership's long-term debt includes the OpCo Revolver and the MLP Revolver at March 31, 2019 . The fair value of debt is determined based on the present value of expected future cash flows using a discounted cash flow methodology. Because the Partnership's valuation methodology used for long-term debt requires the use of significant unobservable inputs, the inputs used to measure the fair value of the Partnership's long-term debt are classified as Level 3 within the fair value hierarchy. Inputs used to estimate the fair values of the Partnership's long-term debt include the selection of an appropriate discount rate. March 31, 2019 December 31, 2018 Carrying Value Fair Value Carrying Value Fair Value OpCo Revolver $ 224,064 $ 225,253 $ 224,064 $ 221,002 MLP Revolver 377,055 375,365 253,544 249,747

Supplemental Information

Supplemental Information	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Supplemental Information	Supplemental Information Accrued Liabilities Accrued liabilities were $20,433 and $16,250 at March 31, 2019 and December 31, 2018 , respectively. Accrued interest, accrued maintenance, capital expenditures and accrued taxes, which are components of accrued liabilities, were $5,852 , $3,850 , $2,827 and $2,931 , respectively, at March 31, 2019 , and $5,448 , $2,688 , $2,818 and $1,564 , respectively, at December 31, 2018 . No other component of accrued liabilities was more than five percent of total current liabilities. Non-cash Investing Activity The change in capital expenditure accrual resulted in an increase in additions to property, plant and equipment by $52 for the three months ended March 31, 2019 . The change in capital expenditure accrual resulted in a decrease in additions to property, plant and equipment by $117 for the three months ended March 31, 2018 .

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Partnership is subject to environmental laws and regulations that can impose civil and criminal sanctions and that may require the Partnership to mitigate the effects of contamination caused by the release or disposal of hazardous substances into the environment. These laws include the federal Clean Air Act, the federal Water Pollution Control Act, the Resource Conservation and Recovery Act ("RCRA"), the Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA"), the Toxic Substances Control Act and various other federal, state and local laws and regulations. Under CERCLA, an owner or operator of property may be held strictly liable for remediating contamination without regard to whether that person caused the contamination, and without regard to whether the practices that resulted in the contamination were legal at the time they occurred. Because the Partnership's production sites have a history of industrial use, it is impossible to predict precisely what effect these legal requirements will have on the Partnership. Westlake will indemnify the Partnership for liabilities that occurred or existed prior to August 4, 2014. The Partnership is involved in various legal proceedings incidental to the conduct of its business. The Partnership does not believe that any of these legal proceedings will have a material adverse effect on its financial condition, results of operations or cash flows.

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events In April 2019, the Partnership repaid $201,445 of borrowings under the OpCo Revolver.

Description of Business and B_2

Description of Business and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated interim financial statements were prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the "SEC") for interim periods. Accordingly, certain information and footnotes required for complete financial statements under generally accepted accounting principles in the United States ("U.S. GAAP") have not been included. These interim consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto of the Partnership included in the annual report on Form 10-K for the fiscal year ended December 31, 2018 (the " 2018 Form 10-K"), filed with the SEC on March 1, 2019 . These financial statements have been prepared in conformity with the accounting principles and practices as disclosed in the notes to the consolidated financial statements of the Partnership for the fiscal year ended December 31, 2018 with the exceptions of those accounting standards adopted in 2019 as discussed in Note 1 . References to "Westlake" refer collectively to Westlake Chemical Corporation and its subsidiaries, other than the Partnership, OpCo and OpCo GP. The Partnership holds a 22.8% limited partner interest and the entire non-economic general partner interest in OpCo. The remaining 77.2% limited partner interest in OpCo is owned by Westlake, which has no rights to direct the activities that most significantly impact the economic performance of OpCo. As a result of the fact that substantially all of OpCo's activities are conducted on behalf of Westlake, and the fact that OpCo exhibits disproportionality of voting rights to economic interest, OpCo was deemed to be a variable interest entity. The Partnership, through its ownership of OpCo's general partner, has the power to direct the activities that most significantly impact the economic performance of OpCo, and it also has the obligation or right to absorb losses or receive benefits from OpCo that could potentially be significant to OpCo. As such, the Partnership was determined to be OpCo's primary beneficiary and therefore consolidates OpCo's results of operations and financial position. Westlake's retained interest of 77.2% is recorded as noncontrolling interest in the Partnership's consolidated financial statements. In the opinion of the Partnership's management, the accompanying unaudited consolidated interim financial statements reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair statement of the Partnership's financial position as of March 31, 2019 , its results of operations for the three months ended March 31, 2019 and 2018 and the changes in its cash position for the three months ended March 31, 2019 and 2018 . Results of operations and changes in cash position for the interim periods presented are not necessarily indicative of the results that will be realized for the fiscal year ending December 31, 2019 or any other interim period. The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities. Actual results could differ materially from those estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Credit Losses (ASU No. 2016-13) In June 2016, the FASB issued an accounting standards update providing new guidance for the accounting for credit losses on loans and other financial instruments. The new guidance introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments. The standard also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. The accounting standard will be effective for reporting periods beginning after December 15, 2019 and is not expected to have a material impact on the Partnership's consolidated financial position, results of operations and cash flows. Fair Value Measurement (ASU No. 2018-13) In August 2018, the FASB issued an accounting standards update to modify the disclosure requirements on fair value measurements. The amendments are effective for reporting periods beginning after December 15, 2019. An entity is permitted to early adopt any removed or modified disclosures and delay adoption of the additional disclosures until the effective date. Most amendments should be applied retrospectively but certain amendments should be applied prospectively. The Partnership is in the process of evaluating the impact that the new accounting guidance will have on the Partnership's consolidated financial position, results of operations and cash flows. Recently Adopted Accounting Standards Leases (ASU No. 2016-02) In February 2016, the FASB issued an accounting standards update on lease accounting that supersedes the previously issued lease guidance. The new standard requires lessees to recognize assets and liabilities for all long-term operating leases. An asset is recognized for the right to use an underlying leased asset and a liability is recognized for the obligation to make payments over the lease term. The standard also requires expanded lease disclosures. The standard requires a modified retrospective adoption approach and allows for the election of certain transition expedients. The Partnership adopted the standard effective January 1, 2019 using the optional transition method which allows entities to recognize a cumulative adjustment to the opening balance sheet in the period of adoption. The Partnership elected the package of optional transition expedients and was not required to reassess (1) whether any existing contracts are or contain leases, (2) classification of existing leases as operating or capital or (3) whether initial direct costs for existing leases qualify for capitalization under the new accounting standard. The Partnership did not elect the use of hindsight to determine the lease term when considering lease renewal or termination options. Adoption of the new standard did not have a material impact on the Partnership's consolidated financial position, results of operations and cash flows.

Accounts Receivable - Third P_2

Accounts Receivable - Third Parties (Tables)	3 Months Ended
	Mar. 31, 2019
Accounts Receivable, Net [Abstract]
Schedule Of Accounts Receivable - Third Parties	Accounts receivable—third parties consist of the following: March 31, December 31, Trade customers $ 14,455 $ 17,325 Allowance for doubtful accounts (728 ) (921 ) 13,727 16,404 Other 1,768 — Accounts receivable, net—third parties $ 15,495 $ 16,404

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Schedule Of Inventory	Inventories consist of the following: March 31, December 31, Finished products $ 3,741 $ 3,876 Feedstock, additives and chemicals 430 512 Inventories $ 4,171 $ 4,388

Distributions and Net Income _2

Distributions and Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Schedule of Incentive Distributions Made to Managing Members or General Partners by Distribution	The distributions are declared subsequent to quarter end; therefore, the table below represents total distributions declared from earnings of the related periods pertaining to such distributions. Three Months Ended March 31, 2019 2018 Net income attributable to the Partnership $ 14,955 $ 12,295 Less: Limited partners' distributions declared on common units 15,666 12,814 Distributions declared with respect to the incentive distribution rights — 733 Net income in excess of distribution (distribution in excess of net income) $ (711 ) $ (1,252 )
Schedule of Earnings Per Share, Basic and Diluted	Net income attributable to the Partnership is allocated to the unitholders in accordance with their respective ownership percentages in preparation of the consolidated statement of equity. However, when distributions related to the incentive distribution rights are made, net income equal to the amount of those distributions is first allocated to the general partner before the remaining net income is allocated to the unitholders based on their respective ownership percentages. Basic and diluted net income per unit is the same because the Partnership does not have any potentially dilutive units outstanding for the periods presented. Three Months Ended March 31, 2019 Limited Partners' Common Units Incentive Distribution Rights Total Net income attributable to the Partnership: Distribution declared $ 15,666 $ — $ 15,666 Distribution in excess of net income (711 ) — (711 ) Net income $ 14,955 $ — $ 14,955 Weighted average units outstanding: Basic and diluted 32,345,398 32,345,398 Net income per limited partner unit: Basic and diluted $ 0.46 Three Months Ended March 31, 2018 Limited Partners' Common Units Incentive Distribution Rights Total Net income attributable to the Partnership: Distribution declared $ 12,814 $ 733 $ 13,547 Distribution in excess of net income (1,252 ) — (1,252 ) Net income $ 11,562 $ 733 $ 12,295 Weighted average units outstanding: Basic and diluted 32,236,449 32,236,449 Net income per limited partner unit: Basic and diluted $ 0.36 The amended Partnership Agreement provides that the Partnership will distribute cash each quarter to all the unitholders, pro rata, until each unit has received a distribution of $1.2938 . If cash distributions to the Partnership's unitholders exceed $1.2938 per unit in any quarter, the Partnership's unitholders and Westlake, as the holder of the Partnership's incentive distribution rights, will receive distributions according to the following percentage allocations: Marginal Percentage Interest in Distributions Total Quarterly Distribution Per Unit Unitholders IDR Holders Above $1.2938 up to $1.4063 85.0 % 15.0 % Above $1.4063 up to $1.6875 75.0 % 25.0 % Above $1.6875 50.0 % 50.0 % The Partnership's distribution for the three months ended March 31, 2019 did not exceed the $1.2938 per unit threshold, and, as a result, no distribution was made with respect to the Partnership's incentive distribution rights to Westlake, as the holder of the Partnership's incentive distribution rights. Distribution Per Common Unit Distributions per common unit for the three months ended March 31, 2019 and 2018 were as follows: Three Months Ended March 31, 2019 2018 Distributions per common unit $ 0.4328 $ 0.3864

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The related party accounts payable balances were as follows: March 31, December 31, Accounts payable—Westlake $ 19,344 $ 27,477 Charges from related parties for goods and services capitalized as assets were as follows: Three Months Ended March 31, 2019 2018 Goods and services purchased from Westlake and capitalized as assets $ 643 $ 351 Sales to related parties were as follows: Three Months Ended March 31, 2019 2018 Net sales—Westlake $ 257,040 $ 235,031 Charges from related parties in cost of sales were as follows: Three Months Ended March 31, 2019 2018 Feedstock purchased from Westlake and included in cost of sales $ 122,155 $ 105,415 Other charges from Westlake and included in cost of sales 27,572 28,406 Total $ 149,727 $ 133,821 Charges from related parties included within selling, general and administrative expenses were as follows: Three Months Ended March 31, 2019 2018 Services received from Westlake and included in selling, general and administrative expenses $ 6,603 $ 6,251 The Partnership's accounts receivable from Westlake were as follows: March 31, December 31, Accounts receivable—Westlake $ 47,626 $ 57,280 Debt payable to related parties was as follows: March 31, December 31, Long-term debt payable to Westlake $ 601,119 $ 477,608 The Partnership's receivable under the Investment Management Agreement was as follows: March 31, December 31, Receivable under the Investment Management Agreement $ 276,501 $ 148,956

Long-term Debt Payable to Wes_2

Long-term Debt Payable to Westlake (Tables)	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions, Long-term Debt	Long-term debt payable to Westlake consists of the following: March 31, December 31, OpCo Revolver (variable interest rate of LIBOR plus 2.0%, scheduled maturity of September 25, 2023) $ 224,064 $ 224,064 MLP Revolver (variable interest rate of LIBOR plus 2.0%, scheduled maturity of April 29, 2021) 377,055 253,544 $ 601,119 $ 477,608

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Summary Of Carrying And Fair Values Of Long-Term Debt	The carrying and fair values of the Partnership's long-term debt at March 31, 2019 and December 31, 2018 are summarized in the table below. The Partnership's long-term debt includes the OpCo Revolver and the MLP Revolver at March 31, 2019 . The fair value of debt is determined based on the present value of expected future cash flows using a discounted cash flow methodology. Because the Partnership's valuation methodology used for long-term debt requires the use of significant unobservable inputs, the inputs used to measure the fair value of the Partnership's long-term debt are classified as Level 3 within the fair value hierarchy. Inputs used to estimate the fair values of the Partnership's long-term debt include the selection of an appropriate discount rate. March 31, 2019 December 31, 2018 Carrying Value Fair Value Carrying Value Fair Value OpCo Revolver $ 224,064 $ 225,253 $ 224,064 $ 221,002 MLP Revolver 377,055 375,365 253,544 249,747

Description of Business and B_3

Description of Business and Basis of Presentation (Details) $ / shares in Units, $ in Thousands	Mar. 29, 2019USD ($)$ / sharesshares	Sep. 29, 2017USD ($)$ / sharesshares	Apr. 29, 2015USD ($)	Apr. 01, 2015	Aug. 04, 2014shares	Mar. 31, 2019USD ($)production_facility	Mar. 31, 2018USD ($)
Limited Partners' Capital Account [Line Items]
Proceeds from private placement of common units	$ 62,934					$ 62,934	$ 0
Westlake Chemical OpCo LP [Member] | Affiliated Entity [Member]
Limited Partners' Capital Account [Line Items]
Limited partner interest	22.80%	18.30%		13.30%	10.60%
Limited partner interest, additional ownership	4.50%	5.00%		2.70%
Amount paid to purchase additional limited partner interest	$ 201,445	$ 229,207	$ 135,341
Westlake Chemical OpCo GP LLC [Member] | Limited Liability Company [Member]
Limited Partners' Capital Account [Line Items]
Limited partner interest					100.00%
Westlake [Member] | Westlake Chemical OpCo LP [Member] | Majority-Owned Subsidiary, Unconsolidated [Member]
Limited Partners' Capital Account [Line Items]
Limited partner interest	77.20%
Westlake Chemical OpCo LP [Member]
Limited Partners' Capital Account [Line Items]
Number of ethylene production facilities | production_facility						3
Limited Partner [Member]
Limited Partners' Capital Account [Line Items]
Number of units sold in public offering | shares	2,940,818	5,175,000			12,937,500
Units sold in secondary offering price per unit (in dollars per share) | $ / shares	$ 21.40	$ 22

Accounts Receivable - Third P_3

Accounts Receivable - Third Parties Accounts Receivable—Third Parties (Schedule Of Accounts Receivable) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounts Receivable, Net [Abstract]
Trade customers	$ 14,455	$ 17,325
Allowance for doubtful accounts	(728)	(921)
Trade Customer Receivable Net Current	13,727	16,404
Other	1,768	0
Accounts receivable, net—third parties	$ 15,495	$ 16,404

Inventories (Details)

Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
Finished products	$ 3,741	$ 3,876
Feedstock, additives and chemicals	430	512
Inventories	$ 4,171	$ 4,388

Property, Plant and Equipment (

Property, Plant and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense on property, plant and equipment	$ 22,214	$ 22,157

Deferred Charges and Other As_2

Deferred Charges and Other Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Deferred Charges and Other Assets [Member]
Other Assets [Line Items]
Amortization expense	$ 4,630	$ 5,541

Distributions and Net Income _3

Distributions and Net Income Per Limited Partner Unit (Distributions Declared) (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 30, 2019	Mar. 31, 2019	Mar. 31, 2018
Distribution Made to Limited Partner [Line Items]
Net income attributable to the Partnership		$ 14,955	$ 12,295
Distributions declared		15,666	13,547
Net income in excess of distribution (distribution in excess of net income)		$ (711)	$ (1,252)
Weighted average limited partner units outstanding (basic and diluted) (in shares)		32,345,398	32,236,449
IDR Holders [Member]
Distribution Made to Limited Partner [Line Items]
Net income attributable to the Partnership		$ 0	$ 733
Distributions declared with respect to the incentive distribution rights		0	733
Net income in excess of distribution (distribution in excess of net income)		0	0
Common units [Member]
Distribution Made to Limited Partner [Line Items]
Net income attributable to the Partnership		14,955	11,562
Distributions declared		15,666	12,814
Net income in excess of distribution (distribution in excess of net income)		$ (711)	$ (1,252)
Weighted average limited partner units outstanding (basic and diluted) (in shares)		32,345,398	32,236,449
Income per limited partner unit (usd per share)		$ 0.46	$ 0.36
Cash Distribution [Member] | IDR Holders [Member]
Distribution Made to Limited Partner [Line Items]
Distributions declared with respect to the incentive distribution rights		$ 0	$ 733
Cash Distribution [Member] | Common units [Member]
Distribution Made to Limited Partner [Line Items]
Distributions declared		$ 15,666	$ 12,814
Cash Distribution [Member] | Common And Subordinated Units [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		$ 1.2938
Cash Distribution [Member] | Common And Subordinated Units [Member] | Subsequent Event [Member]
Distribution Made to Limited Partner [Line Items]
Distribution declared per unit (in usd per unit)	$ 0.4452
Above $1.2938 up to $1.4063 | Minimum [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		1.2938
Above $1.2938 up to $1.4063 | Maximum [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		$ 1.4063
Above $1.2938 up to $1.4063 | Unit Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		85.00%
Above $1.2938 up to $1.4063 | IDR Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		15.00%
Above $1.4063 up to $1.6875 | Minimum [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		$ 1.4063
Above $1.4063 up to $1.6875 | Maximum [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		$ 1.6875
Above $1.4063 up to $1.6875 | Unit Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		75.00%
Above $1.4063 up to $1.6875 | IDR Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		25.00%
Above $1.6875 | Minimum [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution Rights, Target Distribution Per Unit Requiring Marginal Percentage Distribution to IDR Holders		$ 1.6875
Above $1.6875 | Unit Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		50.00%
Above $1.6875 | IDR Holders [Member]
Distribution Made to Limited Partner [Line Items]
Incentive Distribution, Marginal Percentage Interest in Distributions		50.00%

Distributions and Net Income _4

Distributions and Net Income Per Limited Partner Unit (Income In Excess Of Distribution) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Net income attributable to the Partnership	$ 14,955	$ 12,295
Distributions declared	15,666	13,547
Net income in excess of distribution (distribution in excess of net income)	(711)	(1,252)
IDR Holders [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Net income attributable to the Partnership	0	733
Distributions declared with respect to the incentive distribution rights	0	733
Net income in excess of distribution (distribution in excess of net income)	0	0
Common units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Net income attributable to the Partnership	14,955	11,562
Distributions declared	15,666	12,814
Net income in excess of distribution (distribution in excess of net income)	(711)	(1,252)
Cash Distribution [Member] | IDR Holders [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared with respect to the incentive distribution rights	0	733
Cash Distribution [Member] | Common units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared	$ 15,666	$ 12,814

Distributions and Net Income _5

Distributions and Net Income Per Limited Partner Unit (Basic and Diluted Income Per Limited Partner Unit) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Limited Partners' Capital Account [Line Items]
Distributions declared	$ 15,666	$ 13,547
Net income in excess of distribution (distribution in excess of net income)	(711)	(1,252)
Net income	$ 14,955	$ 12,295
Weighted average units outsanding:
Basic and diluted (shares)	32,345,398	32,236,449
Common units [Member]
Limited Partners' Capital Account [Line Items]
Distributions declared	$ 15,666	$ 12,814
Net income in excess of distribution (distribution in excess of net income)	(711)	(1,252)
Net income	$ 14,955	$ 11,562
Weighted average units outsanding:
Basic and diluted (shares)	32,345,398	32,236,449
Net income per limited partner unit:
Income per limited partner unit (usd per share)	$ 0.46	$ 0.36
IDR Holders [Member]
Limited Partners' Capital Account [Line Items]
Subsequent distribution to IDR holders	$ 0	$ 733
Net income in excess of distribution (distribution in excess of net income)	0	0
Net income	$ 0	$ 733

Distributions and Net Income _6

Distributions and Net Income Per Limited Partner Unit (Distribution Per Common Unit) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Distribution Per Common Unit [Abstract]
Distribution Made to Limited Partner, Distributions Paid, Per Unit	$ 0.4328	$ 0.3864

Partners' Equity (Details)

Partners' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Mar. 29, 2019	Oct. 04, 2018	Sep. 29, 2017	Aug. 04, 2014	Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Partners Units, Maximum Aggregate Offering Amount, ATM		$ 50
Proceeds from private placement of common units	$ 62,934				$ 62,934	$ 0
Limited Partner [Member]
Class of Stock [Line Items]
Number of units sold in public offering	2,940,818		5,175,000	12,937,500
Units sold in secondary offering price per unit (in dollars per share)	$ 21.40		$ 22
Affiliated Entity [Member]
Class of Stock [Line Items]
Partners' Capital Account, Units, Sold to Related Party	1,401,869

Related Party Transactions (Sal

Related Party Transactions (Sales to Related Parties) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transactions [Abstract]
Revenue from Related Parties	$ 257,040	$ 235,031

Related Party Transactions (Cos

Related Party Transactions (Cost of Sales from Related Parties) (Details) - Affiliated Entity [Member] - Westlake [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Charges from related parties in cost of sales	$ 149,727	$ 133,821
Feedstock purchased [Member]
Related Party Transaction [Line Items]
Charges from related parties in cost of sales	122,155	105,415
Other service charges [Member]
Related Party Transaction [Line Items]
Charges from related parties in cost of sales	$ 27,572	$ 28,406

Related Party Transactions (Ser

Related Party Transactions (Services from Related Parties Included in SG&A Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Services received from Westlake and included in selling, general and administrative expenses	$ 6,603	$ 6,251

Related Party Transactions (Goo

Related Party Transactions (Goods and Services from Related Parties that have been Capitalized) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Goods and services purchased from Westlake and capitalized as assets	$ 643	$ 351

Related Party Transactions (Acc

Related Party Transactions (Accounts Receivable from and Accounts Payable to Related Parties) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Receivable under the Investment Management Agreement—Westlake Chemical Corporation (Westlake)	$ 276,501	$ 148,956
Accounts payable—Westlake	19,344	27,477
Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Accounts receivable—Westlake, current and non-current	$ 47,626	$ 57,280

Related Party Transactions (Deb

Related Party Transactions (Debt Payable to Related Parties) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Interest on related party debt payable	$ 5,900	$ 4,866
Accrued interest on related party debt	5,852		$ 5,448
Long-term debt payable to Westlake	601,119		477,608
Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Interest capitalized	0	15
Other income (expense) [Member] | Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Interest on related party debt payable	5,900	$ 4,866
Accrued Liabilities [Member] | Affiliated Entity [Member] | Westlake [Member]
Related Party Transaction [Line Items]
Accrued interest on related party debt	$ 5,852		$ 5,448

Related Party Transactions (Gen

Related Party Transactions (General) (Details) - Affiliated Entity [Member] - Westlake [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Operating Lease, Expense	$ 514	$ 318
Interest earned related to the IMA	818	$ 540
Accrued Liabilities [Member]
Related Party Transaction [Line Items]
Accrued interest included in receivable under IMA	$ 34		$ 496

Related Party Transactions (Sit

Related Party Transactions (Site Lease Agreements) (Details)	3 Months Ended
	Mar. 31, 2019USD ($)
Related Party Transaction [Line Items]
Site lease agreement, number	2
Site lease agreements, term (in years)	50
Site lease agreements, OpCo annual obligation to WLK	$ 1

Related Party Transactions (Maj

Related Party Transactions (Major Customer and Concentration Risk) (Details)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Affiliated Entity [Member] | Westlake [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	85.90%	82.70%

Long-term Debt Payable to Wes_3

Long-term Debt Payable to Westlake (Details) - USD ($) $ in Thousands	Mar. 29, 2019	Sep. 29, 2017	Apr. 01, 2015	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Proceeds from debt payable to Westlake				$ 123,512	$ 3,648
Long-term debt payable to Westlake				$ 601,119		$ 477,608
Weighted average interest rate				4.80%		4.40%
Limited Partner [Member] | OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Long-term debt payable to Westlake				$ 224,064		$ 224,064
Limited Partner [Member] | OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent				2.00%		2.00%
Limited Partner [Member] | MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Long-term debt payable to Westlake				$ 377,055		$ 253,544
Limited Partner [Member] | MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent				2.00%		2.00%
Westlake Chemical OpCo LP [Member] | Affiliated Entity [Member]
Related Party Transaction [Line Items]
Proceeds from debt payable to Westlake	$ 123,512
Limited partner interest, additional ownership	4.50%	5.00%	2.70%

Fair Value Measurements (Summar

Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	$ 601,119	$ 477,608
OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, fair value	225,253	221,002
OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | Limited Partner [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	224,064	224,064
MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, fair value	375,365	249,747
MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | Limited Partner [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	$ 377,055	$ 253,544

Supplemental Information (Detai

Supplemental Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accrued Liabilities [Abstract]
Accrued and other liabilities	$ 20,433		$ 16,250
Accrued interest	5,852		5,448
Accrued maintenance	3,850		2,688
Accrued Income Taxes	2,931		1,564
Accrued Capital Expenditures, Current	2,827		$ 2,818
Increase (reduction) in capital expenditure accrual	$ 52	$ 117

Subsequent Events (Details)

Subsequent Events (Details)	1 Months Ended
	Apr. 30, 2019USD ($)
Subsequent Event [Member]
Subsequent Event [Line Items]
Repayments of Debt	$ 201,445,000