CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Common Stock, Shares, Outstanding		199.7
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Balance at Dec. 31, 2016	$ 3,443.8	$ 2.0	$ 1,103.8	$ 2,367.3	$ (29.3)
Comprehensive income (loss)
Net income	619.0	$ 0.0	0.0	619.0	0.0
Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification, Number of Shares		0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Other comprehensive income, net of tax	10.6	$ 0.0	0.0	0.0	10.6
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	22.3	$ 0.0	0.0	26.8	(4.5)
Dividends, Common Stock, Cash, Number of Shares		0.0
Dividends declared on common stock	(253.6)	$ 0.0	0.0	(253.6)	0.0
Stock Issued During Period, Shares, New Issues		0.0
Expense of common stock	(0.1)	$ 0.0	(0.1)	0.0	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	9.1	$ 0.0	9.1	0.0	0.0
Balance at Dec. 31, 2017	$ 3,851.1	$ 2.0	1,112.8	2,759.5	(23.2)
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.2700
Common Stock, Shares, Outstanding		199.7
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Net income	$ 425.5	$ 0.0	0.0	425.5	0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Other comprehensive income, net of tax	(5.7)	$ 0.0	0.0	0.0	(5.7)
Dividends, Common Stock, Cash, Number of Shares		0.0
Dividends declared on common stock	(278.7)	$ 0.0	0.0	(278.7)	0.0
Stock Issued During Period, Shares, New Issues		0.0
Expense of common stock	(0.1)	$ 0.0	(0.1)	0.0	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	13.0	$ 0.0	13.0	0.0	0.0
Balance at Dec. 31, 2018	$ 4,005.1	$ 2.0	1,125.7	2,906.3	(28.9)
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.3950
Common Stock, Shares, Outstanding	199.7	199.7
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Net income	$ 433.6	$ 0.0	0.0	433.6	0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Other comprehensive income, net of tax	1.0	$ 0.0	0.0	0.0	1.0
Dividends, Common Stock, Cash, Number of Shares		0.0
Dividends declared on common stock	(303.8)	$ 0.0	0.0	(303.8)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.4
Stock-based compensation	3.6	$ 0.0	3.6	0.0	0.0
Balance at Dec. 31, 2019	$ 4,139.5	$ 2.0	$ 1,129.3	$ 3,036.1	$ (27.9)
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.5050
Common Stock, Shares, Outstanding	200.1	200.1