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, 2015	$ 3,326.0	$ 2.0	$ 1,099.3	$ 2,259.8	$ (35.1)
Comprehensive income (loss)
Net income	338.2	$ 0.0	0.0	338.2	0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Other comprehensive loss, net of tax	5.8	$ 0.0	0.0	0.0	5.8
Dividends declared on common stock		0.0
Dividends declared on common stock	(230.7)	$ 0.0	0.0	(230.7)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	4.5	$ 0.0	4.5	0.0	0.0
Balance at Dec. 31, 2016	3,443.8	$ 2.0	1,103.8	2,367.3	(29.3)
Common Stock, Shares, Outstanding		199.7
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	22.3	$ 0.0	0.0	26.8	(4.5)
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
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(4.5)
Other comprehensive loss, net of tax	10.6	$ 0.0	0.0	0.0	10.6
Dividends declared on common stock		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)
Common Stock, Shares, Outstanding	199.7	199.7
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Comprehensive income (loss)
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 loss, net of tax	(5.7)	$ 0.0	0.0	0.0	(5.7)
Dividends declared on common stock		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)
Common Stock, Shares, Outstanding	199.7	199.7