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, 2021		$ 692.5	$ 0.2		$ 1,078.9	$ 93.0	$ (475.9)	$ (3.7)
Beginning balance (shares) at Dec. 31, 2021			15,865,118
Net income		8.1				8.1
Other comprehensive income, net of tax		18.0						18.0
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			58,641
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued		(1.9)			(1.9)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)			(19,643)
Cash dividends declared	[1]	(12.5)				(12.5)
Amortization of unearned equity compensation		4.0			4.0
Ending balance at Mar. 31, 2022		708.2	$ 0.2		1,081.0	88.6	(475.9)	14.3
Ending balance (shares) at Mar. 31, 2022			15,904,116
Beginning balance at Dec. 31, 2021		692.5	$ 0.2		1,078.9	93.0	(475.9)	(3.7)
Beginning balance (shares) at Dec. 31, 2021			15,865,118
Net income		(3.2)
Other comprehensive income, net of tax		(18.0)
Ending balance at Sep. 30, 2022		642.0	$ 0.2		1,087.2	52.2	(475.9)	(21.7)
Ending balance (shares) at Sep. 30, 2022			15,940,580
Beginning balance at Mar. 31, 2022		708.2	$ 0.2		1,081.0	88.6	(475.9)	14.3
Beginning balance (shares) at Mar. 31, 2022			15,904,116
Net income		(13.8)				(13.8)
Other comprehensive income, net of tax		(35.6)						(35.6)
Common shares issued (including impacts from Long-Term Incentive programs)		0.6			0.6
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			18,101
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)			(155)
Cash dividends declared	[1]	(12.6)				(12.6)
Amortization of unearned equity compensation		3.2			3.2
Ending balance at Jun. 30, 2022		650.0	$ 0.2		1,084.8	62.2	(475.9)	(21.3)
Ending balance (shares) at Jun. 30, 2022			15,922,062
Net income		2.5				2.5
Other comprehensive income, net of tax		(0.4)						(0.4)
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			30,476
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued		(0.9)			(0.9)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)			(11,958)
Cash dividends declared	[1]	(12.5)				(12.5)
Amortization of unearned equity compensation		3.3			3.3
Ending balance at Sep. 30, 2022		642.0	$ 0.2		1,087.2	52.2	(475.9)	(21.7)
Ending balance (shares) at Sep. 30, 2022			15,940,580
Beginning balance at Dec. 31, 2022		$ 631.2	$ 0.2		1,090.4	13.3	(475.9)	3.2
Beginning balance (shares) at Dec. 31, 2022		15,940,756	15,940,756	[2]
Net income		$ 15.9				15.9
Other comprehensive income, net of tax		(1.1)						(1.1)
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[2]		49,128
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued		(1.3)			(1.3)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)	[2]		(15,848)
Cash dividends declared	[3]	(12.5)				(12.5)
Amortization of unearned equity compensation		3.4			3.4
Ending balance at Mar. 31, 2023		635.6	$ 0.2		1,092.5	16.7	(475.9)	2.1
Ending balance (shares) at Mar. 31, 2023	[2]		15,974,036
Beginning balance at Dec. 31, 2022		$ 631.2	$ 0.2		1,090.4	13.3	(475.9)	3.2
Beginning balance (shares) at Dec. 31, 2022		15,940,756	15,940,756	[2]
Net income		$ 39.6
Other comprehensive income, net of tax		(2.4)
Ending balance at Sep. 30, 2023		$ 640.7	$ 0.2		1,100.4	15.2	(475.9)	0.8
Ending balance (shares) at Sep. 30, 2023		16,013,818	16,013,818	[2]
Beginning balance at Mar. 31, 2023		$ 635.6	$ 0.2		1,092.5	16.7	(475.9)	2.1
Beginning balance (shares) at Mar. 31, 2023	[2]		15,974,036
Net income		18.3				18.3
Other comprehensive income, net of tax		(7.3)						(7.3)
Common shares issued (including impacts from Long-Term Incentive programs)		0.7			0.7
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[2]		45,164
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued		(0.4)			(0.4)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)	[2]		(6,036)
Cash dividends declared	[3]	(12.6)				(12.6)
Amortization of unearned equity compensation		3.8			3.8
Ending balance at Jun. 30, 2023		638.1	$ 0.2		1,096.6	22.4	(475.9)	(5.2)
Ending balance (shares) at Jun. 30, 2023	[2]		16,013,164
Net income		5.4				5.4
Other comprehensive income, net of tax		6.0						6.0
Common shares issued (including impacts from Long-Term Incentive programs) (shares)	[2]		1,000
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued		(0.1)			(0.1)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares/common shares issued (shares)	[2]		(346)
Cash dividends declared	[3]	(12.6)				(12.6)
Amortization of unearned equity compensation		3.9			3.9
Ending balance at Sep. 30, 2023		$ 640.7	$ 0.2		$ 1,100.4	$ 15.2	$ (475.9)	$ 0.8
Ending balance (shares) at Sep. 30, 2023		16,013,818	16,013,818	[2]

[1] Dividends declared per common share were $0.77 for the quarters ended March 31, 2022, June 30, 2022 and September 30, 2022.

[2] At September 30, 2023, 506,918 shares were available for awards under the Kaiser Aluminum Corporation 2021 Equity and Incentive Compensation Plan (“2021 Plan”)

[3] Dividends declared per common share were $0.77 for the quarters ended March 31, 2023, June 30, 2023 and September 30, 2023.