Consolidated Statements of Stockholders' Deficit - USD ($) $ in Thousands	Common stock Cumulative Effect, Period of Adoption, Adjusted Balance	Common stock	Capital in excess of par value Cumulative Effect, Period of Adoption, Adjusted Balance	Capital in excess of par value	Accumulated deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated deficit Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated deficit	Accumulated other comprehensive loss Cumulative Effect, Period of Adoption, Adjustment	Accumulated other comprehensive loss Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated other comprehensive loss	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Total
Beginning balance (Accounting Standards Update 2014-09 and 2018-02) at Dec. 31, 2017					$ 4,557			$ (9,439)			$ (4,882)
Beginning balance at Dec. 31, 2017	$ 2,112	$ 2,112	$ 1,086,265	$ 1,086,265		$ (1,525,051)	$ (1,529,608)		$ (73,320)	$ (63,881)		$ (509,994)	$ (505,112)
Beginning balance (in shares) at Dec. 31, 2017	84,488,433	84,488,433
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock		$ 31		41,626									41,657
Issuance of common stock (in shares)		1,264,497
Stock-based compensation				(46,684)									(46,684)
Dividends declared to common shareholders							(265,755)						(265,755)
Repurchase of common stock		$ (45)		(14,341)			(96,604)						(110,990)
Repurchase of common stock (in shares)		(1,827,991)
Employee stock purchase plan		$ 1		2,047									2,048
Employee stock purchase plan (in shares)		37,243
Fresh start valuation adjustment for partnership park units purchased							80						80
Change in redemption value of partnership units				(31,273)									(31,273)
Net income (loss) attributable to Six Flags Entertainment Corporation							275,996						275,996
Other comprehensive income (loss), net of tax										1,822			1,822
Ending balance at Dec. 31, 2018		$ 2,099		1,037,640			(1,611,334)			(71,498)			(643,093)
Ending balance (in shares) at Dec. 31, 2018		83,962,182
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock		$ 16		17,480									17,496
Issuance of common stock (in shares)		622,787
Stock-based compensation				13,274									13,274
Dividends declared to common shareholders							(277,523)						(277,523)
Repurchase of common stock				(52)									(52)
Repurchase of common stock (in shares)		(882)
Employee stock purchase plan		$ 1		2,130									2,131
Employee stock purchase plan (in shares)		49,758
Fresh start valuation adjustment for partnership park units purchased							45						45
Change in redemption value of partnership units				(4,249)									(4,249)
Net income (loss) attributable to Six Flags Entertainment Corporation							179,065						179,065
Other comprehensive income (loss), net of tax										(3,212)			(3,212)
Ending balance at Dec. 31, 2019		$ 2,116		1,066,223			(1,709,747)			(74,710)			(716,118)
Ending balance (in shares) at Dec. 31, 2019		84,633,845
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock		$ 8		2,237									2,245
Issuance of common stock (in shares)		371,182
Stock-based compensation				19,530									19,530
Dividends declared to common shareholders							(21,165)						(21,165)
Repurchase of common stock				(54)									(54)
Repurchase of common stock (in shares)		(2,291)
Employee stock purchase plan		$ 2		1,281									1,283
Employee stock purchase plan (in shares)		73,165
Fresh start valuation adjustment for partnership park units purchased							924						924
Change in redemption value of partnership units				(18)									(18)
Net income (loss) attributable to Six Flags Entertainment Corporation							(423,380)						(423,380)
Other comprehensive income (loss), net of tax										(21,794)			(21,794)
Ending balance at Dec. 31, 2020		$ 2,126		$ 1,089,199			$ (2,153,368)			$ (96,504)			$ (1,158,547)
Ending balance (in shares) at Dec. 31, 2020		85,075,901