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, 2018		$ 740.4	$ 0.2		$ 1,059.3	$ 150.2	$ (420.5)	$ (48.8)
Beginning balance (shares) at Dec. 31, 2018			16,234,603
Stockholders' Equity [Roll Forward]
Net income (loss)		28.0				28.0
Other comprehensive income (loss), net of tax		9.7						9.7
Common shares issued (including impacts from Long-Term Incentive programs) (shares)			122,309
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(5.0)			(5.0)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)			(46,388)
Repurchase of common stock	[1]	(17.4)					(17.4)
Repurchase of common stock (shares)	[1]		(175,977)
Cash dividends declared	[2]	(10.2)				(10.2)
Amortization of unearned equity compensation		2.4			2.4
Ending balance at Mar. 31, 2019		747.9	$ 0.2		1,056.7	168.0	(437.9)	(39.1)
Ending balance (shares) at Mar. 31, 2019			16,134,547
Beginning balance at Dec. 31, 2018		740.4	$ 0.2		1,059.3	150.2	(420.5)	(48.8)
Beginning balance (shares) at Dec. 31, 2018			16,234,603
Stockholders' Equity [Roll Forward]
Net income (loss)		72.6
Other comprehensive income (loss), net of tax		5.6
Ending balance at Sep. 30, 2019		751.4	$ 0.2		1,061.5	193.0	(460.1)	(43.2)
Ending balance (shares) at Sep. 30, 2019			15,900,956
Beginning balance at Mar. 31, 2019		747.9	$ 0.2		1,056.7	168.0	(437.9)	(39.1)
Beginning balance (shares) at Mar. 31, 2019			16,134,547
Stockholders' Equity [Roll Forward]
Net income (loss)		19.2				19.2
Other comprehensive income (loss), net of tax		(3.2)						(3.2)
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)			14,700
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(1.2)			(1.2)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)			(12,395)
Repurchase of common stock	[1]	(10.2)					(10.2)
Repurchase of common stock (shares)	[1]		(108,177)
Cash dividends declared	[2]	(9.9)				(9.9)
Amortization of unearned equity compensation		3.4			3.4
Ending balance at Jun. 30, 2019		746.3	$ 0.2		1,059.2	177.3	(448.1)	(42.3)
Ending balance (shares) at Jun. 30, 2019			16,028,675
Stockholders' Equity [Roll Forward]
Net income (loss)		25.4				25.4
Other comprehensive income (loss), net of tax		(0.9)						(0.9)
Issuance of non-vested shares to non-employee directors (shares)			896
Repurchase of common stock	[1]	(12.0)					(12.0)
Repurchase of common stock (shares)	[1]		(128,615)
Cash dividends declared	[2]	(9.7)				(9.7)
Amortization of unearned equity compensation		2.3			2.3
Ending balance at Sep. 30, 2019		751.4	$ 0.2		1,061.5	193.0	(460.1)	(43.2)
Ending balance (shares) at Sep. 30, 2019			15,900,956
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	15,868,304	[3]
Stockholders' Equity [Roll Forward]
Net income (loss)		$ 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)	[3]		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)	[3]		(45,545)
Repurchase of common stock	[4]	(12.5)					(12.5)
Repurchase of common stock (shares)	[3],[4]		(152,763)
Cash dividends declared	[5]	(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	[3]		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	15,868,304	[3]
Stockholders' Equity [Roll Forward]
Net income (loss)		$ 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	15,812,169	[3]
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	[3]		15,789,594
Stockholders' Equity [Roll Forward]
Net income (loss)		(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)	[3]		22,575
Repurchase of common stock (shares)			0
Cash dividends declared	[5]	(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	[3]		15,812,169
Stockholders' Equity [Roll Forward]
Net income (loss)		0.4				0.4
Other comprehensive income (loss), net of tax		8.2						8.2
Repurchase of common stock (shares)			0
Cash dividends declared	[5]	(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	15,812,169	[3]

[1] Weighted-average repurchase price (dollars per share) for the quarters ended March 31, 2019, June 30, 2019 and September 30, 2019 was $98.55, $95.00 and $92.95, respectively.

[2] Dividends declared per common share were $0.60 for each of the quarters ended March 31, 2019, June 30, 2019 and September 30, 2019.

[3] At September 30, 2020, 309,181 shares were available for awards under the Kaiser Aluminum Corporation 2016 Equity and Incentive Compensation Plan (“2016 Plan”).

[4] Weighted-average repurchase price (dollars per share) for the quarter ended March 31, 2020 was $81.94. There were no repurchases of common stock during the quarters ended June 30, 2020 and September 30, 2020. At September 30, 2020, $93.1 million remained available to repurchase our common shares pursuant to the stock repurchase program.

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