STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions		Total	Common Stock		Additional Paid in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2019		$ 733.9	$ 0.2		$ 1,062.9	$ 172.8	$ (463.4)	$ (38.6)
Beginning balance (shares) at Dec. 31, 2019			15,868,304
Stockholders' Equity [Roll Forward]
Net (loss) income		29.1				29.1
Other comprehensive income (loss), net of tax		(15.5)						(15.5)
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			119,598
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(4.3)			(4.3)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)			(45,545)
Repurchase of common stock	[1]	(12.5)					(12.5)
Repurchase of common stock (shares)	[1]		(152,763)
Cash dividends declared	[2]	(11.3)				(11.3)
Amortization of unearned equity compensation		1.7			1.7
Ending balance at Mar. 31, 2020		721.1	$ 0.2		1,060.3	190.6	(475.9)	(54.1)
Ending balance (shares) at Mar. 31, 2020			15,789,594
Beginning balance at Dec. 31, 2019		733.9	$ 0.2		1,062.9	172.8	(463.4)	(38.6)
Beginning balance (shares) at Dec. 31, 2019			15,868,304
Stockholders' Equity [Roll Forward]
Net (loss) income		22.9
Other comprehensive income (loss), net of tax		4.6
Ending balance at Sep. 30, 2020		719.3	$ 0.2		1,066.0	163.0	(475.9)	(34.0)
Ending balance (shares) at Sep. 30, 2020			15,812,169
Beginning balance at Mar. 31, 2020		721.1	$ 0.2		1,060.3	190.6	(475.9)	(54.1)
Beginning balance (shares) at Mar. 31, 2020			15,789,594
Stockholders' Equity [Roll Forward]
Net (loss) income		(6.6)				(6.6)
Other comprehensive income (loss), net of tax		11.9						11.9
Common shares issued (including impacts from Long-Term Incentive programs)		0.4			0.4
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			22,575
Repurchase of common stock (shares)			0
Cash dividends declared	[2]	(10.6)				(10.6)
Amortization of unearned equity compensation		2.6			2.6
Ending balance at Jun. 30, 2020		718.8	$ 0.2		1,063.3	173.4	(475.9)	(42.2)
Ending balance (shares) at Jun. 30, 2020			15,812,169
Stockholders' Equity [Roll Forward]
Net (loss) income		0.4				0.4
Other comprehensive income (loss), net of tax		8.2						8.2
Repurchase of common stock (shares)			0
Cash dividends declared	[2]	(10.8)				(10.8)
Amortization of unearned equity compensation		2.7			2.7
Ending balance at Sep. 30, 2020		719.3	$ 0.2		1,066.0	163.0	(475.9)	(34.0)
Ending balance (shares) at Sep. 30, 2020			15,812,169
Beginning balance at Dec. 31, 2020		$ 732.4	$ 0.2		1,068.6	158.2	(475.9)	(18.7)
Beginning balance (shares) at Dec. 31, 2020		15,812,169	15,812,169	[3],[4]
Stockholders' Equity [Roll Forward]
Net (loss) income		$ 4.5				4.5
Other comprehensive income (loss), net of tax		9.1						9.1
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[3],[4]		56,394
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(2.4)			(2.4)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)	[3],[4]		(20,625)
Repurchase of common stock (shares)			0
Cash dividends declared	[5]	(11.7)				(11.7)
Amortization of unearned equity compensation		3.1			3.1
Ending balance at Mar. 31, 2021		735.0	$ 0.2		1,069.3	151.0	(475.9)	(9.6)
Ending balance (shares) at Mar. 31, 2021	[3],[4]		15,847,938
Beginning balance at Dec. 31, 2020		$ 732.4	$ 0.2		1,068.6	158.2	(475.9)	(18.7)
Beginning balance (shares) at Dec. 31, 2020		15,812,169	15,812,169	[3],[4]
Stockholders' Equity [Roll Forward]
Net (loss) income		$ (20.2)
Other comprehensive income (loss), net of tax		35.8
Ending balance at Sep. 30, 2021		$ 721.0	$ 0.2		1,076.6	103.0	(475.9)	17.1
Ending balance (shares) at Sep. 30, 2021		15,862,672	15,862,672	[3],[4]
Beginning balance at Mar. 31, 2021		$ 735.0	$ 0.2		1,069.3	151.0	(475.9)	(9.6)
Beginning balance (shares) at Mar. 31, 2021	[3],[4]		15,847,938
Stockholders' Equity [Roll Forward]
Net (loss) income		(22.4)				(22.4)
Other comprehensive income (loss), net of tax		15.9						15.9
Common shares issued (including impacts from Long-Term Incentive programs)		0.3			0.3
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[3],[4]		12,291
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)	[3],[4]		(409)
Repurchase of common stock (shares)			0
Cash dividends declared	[5]	(11.7)				(11.7)
Amortization of unearned equity compensation		3.6			3.6
Ending balance at Jun. 30, 2021		720.7	$ 0.2		1,073.2	116.9	(475.9)	6.3
Ending balance (shares) at Jun. 30, 2021	[3],[4]		15,859,820
Stockholders' Equity [Roll Forward]
Net (loss) income		(2.3)				(2.3)
Other comprehensive income (loss), net of tax		10.8						10.8
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[3],[4]		3,431
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(0.1)			(0.1)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)	[3],[4]		(579)
Repurchase of common stock (shares)			0
Cash dividends declared	[5]	(11.6)				(11.6)
Amortization of unearned equity compensation		3.5			3.5
Ending balance at Sep. 30, 2021		$ 721.0	$ 0.2		$ 1,076.6	$ 103.0	$ (475.9)	$ 17.1
Ending balance (shares) at Sep. 30, 2021		15,862,672	15,862,672	[3],[4]

[1] Weighted-average repurchase price (dollars per share) for the quarter ended March 31, 2020 was $81.94. There was no repurchase of common stock for the quarters ended June 30, 2020 and September 30, 2020.

[2] Dividends declared per common share was $0.67 for each of the quarters ended March 31, 2020, June 30, 2020 and September 30, 2020.

[3] At September 30, 2021, 710,580 shares were available for awards under the Kaiser Aluminum Corporation 2021 Equity and Incentive Compensation Plan (“2021 Plan”).

[4] There were no repurchases of common stock during the quarters ended March 31, 2021, June 30, 2021 and September 30, 2021. At September 30, 2021, $93.1 million remained available to repurchase our common shares pursuant to the stock repurchase program.

[5] Dividends declared per common share was $0.72 for the quarters ended March 31, 2021, June 30, 2021 and September 30, 2021.