CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Common Stock, Shares, Outstanding		199,700
Comprehensive income (loss)
Treasury Stock, Common, Shares						0
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0				0
Balance at Dec. 31, 2017	$ 3,851.1	$ 2.0	$ 1,112.8	$ 2,759.5	$ (23.2)	$ 0.0
Comprehensive income (loss)
Net income (loss)	425.5	$ 0.0	0.0	425.5	0.0	$ 0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0				0
Other Comprehensive Income (Loss), Net of Tax	(5.7)	$ 0.0	0.0	0.0	(5.7)	$ 0.0
Dividends, Common Stock, Cash, Number of Shares		0				0
Dividends declared on common stock	(278.7)	$ 0.0	0.0	(278.7)	0.0	$ 0.0
Stock Issued During Period, Shares, New Issues		0				0
Purchase of treasury stock	(0.1)	$ 0.0	(0.1)	0.0	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	$ 0.0
Balance at Dec. 31, 2018	$ 4,005.1	$ 2.0	1,125.7	2,906.3	(28.9)	$ 0.0
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.3950
Common Stock, Shares, Outstanding		199,700
Treasury Stock, Common, Shares						0
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0				0
Net income (loss)	$ 433.6	$ 0.0	0.0	433.6	0.0	$ 0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0				0
Other Comprehensive Income (Loss), Net of Tax	1.0	$ 0.0	0.0	0.0	1.0	$ 0.0
Dividends, Common Stock, Cash, Number of Shares		0				0
Dividends declared on common stock	(303.8)	$ 0.0	0.0	(303.8)	0.0	$ 0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		400				0
Stock-based compensation	3.6	$ 0.0	3.6	0.0	0.0	$ 0.0
Balance at Dec. 31, 2019	$ 4,139.5	$ 2.0	1,129.3	3,036.1	(27.9)	$ 0.0
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.5050
Common Stock, Shares, Outstanding	200,100	200,100
Treasury Stock, Common, Shares	0					0
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0				0
Net income (loss)	$ (173.7)	$ 0.0	0.0	(173.7)	0.0	$ 0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0				0
Other Comprehensive Income (Loss), Net of Tax	(4.2)	$ 0.0	0.0	0.0	(4.2)	$ 0.0
Dividends, Common Stock, Cash, Number of Shares		0				0
Dividends declared on common stock	(317.8)	$ 0.0	0.0	(317.8)	0.0	$ 0.0
Treasury Stock, Value, Acquired, Cost Method		$ 0.0	0.0	0.0	0.0	$ (14.7)
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0				(300)
Stock-based compensation	2.7	$ 0.0	(6.7)	0.0	0.0	$ 9.4
Balance at Dec. 31, 2020	$ 3,631.8	$ 2.0	$ 1,122.6	$ 2,544.6	$ (32.1)	$ (5.3)
Comprehensive income (loss)
DIVIDENDS DECLARED PER COMMON SHARE	$ 1.5800
Treasury Stock, Shares, Acquired	405	0				400
Common Stock, Shares, Outstanding	200,100	200,100
Treasury Stock, Common, Shares	100					100