STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions		Total	Common Stock		Additional Paid in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive 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 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, 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 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		15,847,938	15,847,938	[3],[4]

[1] Weighted-average repurchase price (dollars per share) for the quarter ended March 31, 2020 was $81.94.

[2] Dividends declared per common share was $0.67 for the quarter ended March 31, 2020.

[3] At March 31, 2021, 232,320 shares were available for awards under the Kaiser Aluminum Corporation 2016 Equity and Incentive Compensation Plan (“2016 Plan”).

[4] There were no repurchases of common stock during the quarter ended March 31, 2021. At March 31, 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 quarter ended March 31, 2021.