CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDER'S EQUITY (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings
Common Stock, Shares, Outstanding		40.4
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Balance at Dec. 31, 2017	$ 3,455.7	$ 100.9	$ 926.3	$ 2,428.5
Changes in Stockholders' Equity
Net income	31.3	$ 0.0	0.0	31.3
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	1.0	$ 0.0	1.0	0.0
Balance at Mar. 31, 2018	3,488.0	100.9	927.3	2,459.8
Balance at Dec. 31, 2017	3,455.7	100.9	926.3	2,428.5
Changes in Stockholders' Equity
Net income	123.3
Balance at Jun. 30, 2018	3,581.1	$ 100.9	928.4	2,551.8
Common Stock, Shares, Outstanding		40.4
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Balance at Mar. 31, 2018	3,488.0	$ 100.9	927.3	2,459.8
Changes in Stockholders' Equity
Net income	92.0	$ 0.0	0.0	92.0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	1.1	$ 0.0	1.1	0.0
Balance at Jun. 30, 2018	3,581.1	$ 100.9	928.4	2,551.8
Common Stock, Shares, Outstanding		40.4
Common Stock, Shares, Outstanding		40.4
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Balance at Dec. 31, 2018	3,603.3	$ 100.9	930.9	2,571.5
Changes in Stockholders' Equity
Net income	19.6	$ 0.0	0.0	19.6
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	1.0	$ 0.0	1.0	0.0
Balance at Mar. 31, 2019	3,623.9	100.9	931.9	2,591.1
Balance at Dec. 31, 2018	3,603.3	100.9	930.9	2,571.5
Changes in Stockholders' Equity
Net income	94.1
Balance at Jun. 30, 2019	3,699.4	$ 100.9	932.9	2,665.6
Common Stock, Shares, Outstanding		40.4
Net Income (Loss), Including portion attributable to noncontrolling interest, Number of Shares		0.0
Balance at Mar. 31, 2019	3,623.9	$ 100.9	931.9	2,591.1
Changes in Stockholders' Equity
Net income	74.5	$ 0.0	0.0	74.5
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition, Number of Shares		0.0
Stock-based compensation	1.0	$ 0.0	1.0	0.0
Balance at Jun. 30, 2019	$ 3,699.4	$ 100.9	$ 932.9	$ 2,665.6
Common Stock, Shares, Outstanding		40.4