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 issued for stock options exercised	$ 0.5	0.0	0.5
Common stock issued for other transactions		0.2	3.1
Stock-based compensation			1.3
Other comprehensive income	29.5			29.5
Net income (loss)	(200.1)				(200.1)
Common stock dividends paid					(63.1)
Balance at Jun. 30, 2020	2,188.9	$ 157.9	2,127.0	(773.9)	677.9
Dividends declared per share of common stock		$ 0.40
Balance at Mar. 31, 2020		$ 157.8	2,122.8	(821.1)	829.6
Common stock issued for stock options exercised		0.0	0.0
Common stock issued for other transactions		0.1	0.3
Stock-based compensation			3.9
Other comprehensive income	47.2			47.2
Net income (loss)	(120.1)				(120.1)
Common stock dividends paid					(31.6)
Balance at Jun. 30, 2020	2,188.9	$ 157.9	2,127.0	(773.9)	677.9
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 issued for stock options exercised	50.2	2.4	47.8
Common stock issued for other transactions		0.1	2.6
Stock-based compensation			(0.3)
Other comprehensive income	35.6			35.6
Net income (loss)	599.4				599.4
Common stock dividends paid					(63.7)
Balance at Jun. 30, 2021	2,074.7	$ 160.5	2,187.9	(654.3)	380.6
Dividends declared per share of common stock		$ 0.40
Balance at Mar. 31, 2021		$ 159.2	2,164.3	(683.7)	56.8
Common stock issued for stock options exercised		1.2	23.3
Common stock issued for other transactions		0.1	1.2
Stock-based compensation			(0.9)
Other comprehensive income	29.4			29.4
Net income (loss)	355.8				355.8
Common stock dividends paid					(32.0)
Balance at Jun. 30, 2021	$ 2,074.7	$ 160.5	$ 2,187.9	$ (654.3)	$ 380.6
Dividends declared per share of common stock		$ 0.20