CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2015	$ 3,326.0	$ 2.0	$ 1,099.3	$ 2,259.8	$ (35.1)
Changes in Stockholders' Equity
Net income	96.7	0.0	0.0	96.7	0.0
Net Income (Loss) Attributable to Parent	96.7
Other comprehensive income, net of tax	5.7	0.0	0.0	0.0	5.7
Dividends declared on common stock	(109.8)	0.0	0.0	(109.8)	0.0
Stock-based compensation	3.2	0.0	3.2	0.0	0.0
Balance at Jun. 30, 2016	3,321.8	2.0	1,102.5	2,246.7	(29.4)
Balance at Dec. 31, 2016	3,443.8	2.0	1,103.8	2,367.3	(29.3)
Changes in Stockholders' Equity
Net income	140.8	0.0	0.0		0.0
Net Income (Loss) Attributable to Parent	140.8			140.8
Other comprehensive income, net of tax	1.4	0.0	0.0	0.0	1.4
Dividends declared on common stock	(120.8)	0.0	0.0	(120.8)	0.0
Stock-based compensation	4.5	0.0	4.5	0.0	0.0
Balance at Jun. 30, 2017	3,492.0	2.0	1,108.3	2,409.6	(27.9)
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 22.3	$ 0.0	$ 0.0	$ 22.3	$ 0.0