Accumulated Other Comprehensive Income (Loss) Changes in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total equity, beginning balance	$ 3,892	$ 3,562	$ 3,202
Other comprehensive income (loss)	133	8	(50)
Comprehensive income attributable to Westlake Chemical Corporation	1,432	407	596
Total equity, ending balance	5,369	3,892	3,562
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total equity, beginning balance	(121)	(129)	(79)
Other comprehensive income (loss) before reclassifications	126	59
Amounts reclassified from accumulated other comprehensive income (loss)	2	(51)
Other comprehensive income (loss)	128	8	(50)
Total equity, ending balance	7	(121)	(129)
Benefits Liability, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total equity, beginning balance	29	(8)
Other comprehensive income (loss) before reclassifications	12	36
Amounts reclassified from accumulated other comprehensive income (loss)	2	1
Other comprehensive income (loss)	14	37
Total equity, ending balance	43	29	(8)
Cumulative Foreign Currency Exchange
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total equity, beginning balance	(150)	(116)
Other comprehensive income (loss) before reclassifications	114	(34)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss)	114	(34)
Total equity, ending balance	(36)	(150)	(116)
Net Unrealized Holding Gains on Investments, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total equity, beginning balance	0	(5)
Other comprehensive income (loss) before reclassifications	0	57
Amounts reclassified from accumulated other comprehensive income (loss)	0	(52)
Other comprehensive income (loss)	0	5
Total equity, ending balance	$ 0	$ 0	$ (5)