Condensed Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance at Dec. 31, 2019		$ 157.7	$ 2,122.1	$ (803.4)	$ 941.1
Common stock repurchased and retired		0.0	0.0
Common stock issued for stock options exercised	$ 0.5	0.0	0.5
Common stock issued for other transactions		0.2	3.1
Stock-based compensation			4.2
Other comprehensive income	75.2			75.2
Net income (loss)	(936.9)				(936.9)
Common stock dividends paid					(94.7)
Balance at Sep. 30, 2020	1,469.1	$ 157.9	2,129.9	(728.2)	(90.5)
Dividends declared per share of common stock		$ 0.60
Balance at Jun. 30, 2020		$ 157.9	2,127.0	(773.9)	677.9
Common stock repurchased and retired		0.0	0.0
Common stock issued for stock options exercised		0.0	0.0
Common stock issued for other transactions		0.0	0.0
Stock-based compensation			2.9
Other comprehensive income	45.7			45.7
Net income (loss)	(736.8)				(736.8)
Common stock dividends paid					(31.6)
Balance at Sep. 30, 2020	1,469.1	$ 157.9	2,129.9	(728.2)	(90.5)
Dividends declared per share of common stock		$ 0.20
Balance at Dec. 31, 2020	1,450.8	$ 158.0	2,137.8	(689.9)	(155.1)
Common stock repurchased and retired	(68.3)	(1.5)	(66.8)
Common stock issued for stock options exercised	58.3	2.8	55.5
Common stock issued for other transactions		0.1	3.1
Stock-based compensation			3.5
Other comprehensive income	46.1			46.1
Net income (loss)	990.1				990.1
Common stock dividends paid					(95.8)
Balance at Sep. 30, 2021	2,387.9	$ 159.4	2,133.1	(643.8)	739.2
Dividends declared per share of common stock		$ 0.60
Balance at Jun. 30, 2021		$ 160.5	2,187.9	(654.3)	380.6
Common stock repurchased and retired		(1.5)	(66.8)
Common stock issued for stock options exercised		0.4	7.7
Common stock issued for other transactions		0.0	0.5
Stock-based compensation			3.8
Other comprehensive income	10.5			10.5
Net income (loss)	390.7				390.7
Common stock dividends paid					(32.1)
Balance at Sep. 30, 2021	$ 2,387.9	$ 159.4	$ 2,133.1	$ (643.8)	$ 739.2
Dividends declared per share of common stock		$ 0.20