CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Changes in Stockholders' Equity
Common Stock, Shares, Outstanding		199.7
Balance at Dec. 31, 2017	$ 3,851.1	$ 2.0	$ 1,112.8	$ 2,759.5	$ (23.2)
Changes in Stockholders' Equity
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Net income	55.0	$ 0.0	0.0	55.0	0.0
Dividends, Common Stock, Cash, Number of Shares		0.0
Other comprehensive income net of tax	0.2	$ 0.0	0.0	0.0	0.2
Dividends declared on common stock ($0.3650 per share)	(66.5)	$ 0.0	0.0	(66.5)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	2.3	$ 0.0	2.3	0.0	0.0
Balance at Mar. 31, 2018	3,842.1	2.0	1,115.1	2,748.0	(23.0)
Balance at Dec. 31, 2017	3,851.1	2.0	1,112.8	2,759.5	(23.2)
Changes in Stockholders' Equity
Net income	165.7			165.7
Other comprehensive income net of tax	0.9
Balance at Jun. 30, 2018	3,890.1	$ 2.0	1,118.1	2,792.3	(22.3)
Changes in Stockholders' Equity
Common Stock, Shares, Outstanding		199.7
Balance at Mar. 31, 2018	3,842.1	$ 2.0	1,115.1	2,748.0	(23.0)
Changes in Stockholders' Equity
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Net income	110.7	$ 0.0	0.0	110.7	0.0
Dividends, Common Stock, Cash, Number of Shares		0.0
Other comprehensive income net of tax	0.7	$ 0.0	0.0	0.0	0.7
Dividends declared on common stock ($0.3650 per share)	(66.4)	$ 0.0	0.0	(66.4)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	3.0	$ 0.0	3.0	0.0	0.0
Balance at Jun. 30, 2018	3,890.1	$ 2.0	1,118.1	2,792.3	(22.3)
Changes in Stockholders' Equity
Common Stock, Shares, Outstanding		199.7
Common Stock, Shares, Outstanding		199.7
Balance at Dec. 31, 2018	4,005.1	$ 2.0	1,125.7	2,906.3	(28.9)
Changes in Stockholders' Equity
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Net income	47.1	$ 0.0	0.0	47.1	0.0
Dividends, Common Stock, Cash, Number of Shares		0.0
Other comprehensive income net of tax	6.8	$ 0.0	0.0	0.0	6.8
Dividends declared on common stock ($0.3650 per share)	(75.6)	$ 0.0	0.0	(75.6)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.5
Stock-based compensation	(7.2)	$ 0.0	(7.2)	0.0	0.0
Balance at Mar. 31, 2019	3,976.2	2.0	1,118.5	2,877.8	(22.1)
Balance at Dec. 31, 2018	4,005.1	2.0	1,125.7	2,906.3	(28.9)
Changes in Stockholders' Equity
Net income	147.3			147.3
Other comprehensive income net of tax	7.8
Balance at Jun. 30, 2019	4,007.2	$ 2.0	1,121.4	2,904.9	(21.1)
Changes in Stockholders' Equity
Common Stock, Shares, Outstanding		200.2
Balance at Mar. 31, 2019	3,976.2	$ 2.0	1,118.5	2,877.8	(22.1)
Changes in Stockholders' Equity
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Other Comprehensive Income (Loss), Net of Tax, Number of Shares		0.0
Net income	100.2	$ 0.0	0.0	100.2	0.0
Dividends, Common Stock, Cash, Number of Shares		0.0
Other comprehensive income net of tax	1.0	$ 0.0	0.0	0.0	1.0
Dividends declared on common stock ($0.3650 per share)	(73.1)	$ 0.0	0.0	(73.1)	0.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	2.9	$ 0.0	2.9	0.0	0.0
Balance at Jun. 30, 2019	$ 4,007.2	$ 2.0	$ 1,121.4	$ 2,904.9	$ (21.1)
Changes in Stockholders' Equity
Common Stock, Shares, Outstanding		200.2