STATEMENTS OF CONSOLIDATED STOCKHOLDERS' EQUITY - USD ($) $ in Millions		Total	Cumulative Effect, Period of Adoption, Adjustment	[1]	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid In Capital	Additional Paid In Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	[1]	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjustment	[1]	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance at Dec. 31, 2017		$ 746.3	$ 10.3		$ 756.6	$ 0.2	$ 0.2	$ 1,055.9	$ 1,055.9	$ 85.5	$ 10.7		$ 96.2	$ (358.6)	$ (358.6)	$ (36.7)	$ (0.4)		$ (37.1)
Beginning balance (shares) at Dec. 31, 2017						16,773,586	16,773,586
Accounting Standards Update [Extensible List]		kalu:AccountingStandardsUpdate201409And201601Member
Net income		$ 91.7								91.7
Other comprehensive income (loss), net of tax		(11.7)														(11.7)
Common shares issued (including impacts from Long-Term Incentive programs)		0.2						0.2
Common shares issued (including impacts from Long-Term Incentive programs) (shares)						146,363
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(6.9)						(6.9)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)						(68,195)
Repurchase of common stock	[2]	(61.9)												(61.9)
Repurchase of common stock (shares)	[2]					(617,151)
Cash dividends declared	[3]	(37.7)								(37.7)
Amortization of unearned equity compensation		10.1						10.1
Ending balance at Dec. 31, 2018		740.4				$ 0.2		1,059.3		150.2				(420.5)		(48.8)
Ending balance (shares) at Dec. 31, 2018						16,234,603
Net income		62.0								62.0
Other comprehensive income (loss), net of tax		10.2														10.2
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)						137,905
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(6.2)						(6.2)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)						(58,783)
Repurchase of common stock	[2]	(42.9)												(42.9)
Repurchase of common stock (shares)	[2]					(445,421)
Cash dividends declared	[3]	(39.4)								(39.4)
Amortization of unearned equity compensation		9.5						9.5
Ending balance at Dec. 31, 2019		$ 733.9				$ 0.2		1,062.9		172.8				(463.4)		(38.6)
Ending balance (shares) at Dec. 31, 2019		15,868,304				15,868,304
Net income		$ 28.8								28.8
Other comprehensive income (loss), net of tax		19.9														19.9
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)						142,173
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	[2]	(12.5)												(12.5)
Repurchase of common stock (shares)	[2]					(152,763)
Cash dividends declared	[3]	(43.4)								(43.4)
Amortization of unearned equity compensation		9.6						9.6
Ending balance at Dec. 31, 2020		$ 732.4				$ 0.2		$ 1,068.6		$ 158.2				$ (475.9)		$ (18.7)
Ending balance (shares) at Dec. 31, 2020		15,812,169				15,812,169

[1] Cumulative-effect adjustment relates to our adoption of Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) and ASU No. 2016-01, Financial Instruments – Overall (Subtopic 825-10).

[2] Weighted-average repurchase price (dollars per share) for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 was $81.94, $96.18 and $100.28, respectively. At December 31, 2020, $93.1 million remained available to repurchase our common shares pursuant to the stock repurchase program.

[3] Dividends declared per common share were $2.68, $2.40 and $2.20 during 2020, 2019 and 2018, respectively.