STATEMENTS OF CHANGES IN STOCKHOLDER'S EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings
Balance, shares		40.4
Dividends declared on common stock, shares		0.0
Net income, shares		0.0
Balance at Dec. 31, 2016	$ 3,252.1	$ 100.9	$ 923.2	$ 2,228.0
Comprehensive income (loss)
Net income	305.5	0.0	0.0	305.5
Dividends declared on common stock	(105.0)	$ 0.0	0.0	(105.0)
Stock-based compensation, shares		0.0
Stock-based compensation	3.1	$ 0.0	3.1	0.0
Balance at Dec. 31, 2017	3,455.7	$ 100.9	926.3	2,428.5
Balance, shares		40.4
Dividends declared on common stock, shares		0.0
Net income, shares		0.0
Comprehensive income (loss)
Net income	328.0	$ 0.0	0.0	328.0
Dividends declared on common stock	(185.0)	$ 0.0	0.0	(185.0)
Stock-based compensation, shares		0.0
Stock-based compensation	4.6	$ 0.0	4.6	0.0
Balance at Dec. 31, 2018	$ 3,603.3	$ 100.9	930.9	2,571.5
Balance, shares	40.4	40.4
Net income, shares		0.0
Comprehensive income (loss)
Net income	$ 350.2	$ 0.0	0.0	350.2
Dividends declared on common stock				0.0
Stock-based compensation, shares		0.0
Stock-based compensation	4.8	$ 0.0	4.8	0.0
Balance at Dec. 31, 2019	$ 3,958.3	$ 100.9	$ 935.7	$ 2,921.7
Balance, shares	40.4	40.4