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	25.2	0.0	0.0	25.2	0.0
Other comprehensive income, net of tax	0.4	0.0	0.0	0.0	0.4
Dividends declared on common stock	(54.9)	0.0	0.0	(54.9)	0.0
Stock-based compensation	1.6	0.0	1.6	0.0	0.0
Balance at Mar. 31, 2016	3,298.3	2.0	1,100.9	2,230.1	(34.7)
Balance at Dec. 31, 2016	3,443.8	2.0	1,103.8	2,367.3	(29.3)
Changes in Stockholders' Equity
Net income	36.0	0.0	0.0	36.0	0.0
Other comprehensive income, net of tax	0.6	0.0	0.0	0.0	0.6
Dividends declared on common stock	(60.4)	0.0	0.0	(60.4)	0.0
Stock-based compensation	2.1	0.0	2.1	0.0	0.0
Balance at Mar. 31, 2017	3,444.4	2.0	1,105.9	2,365.2	(28.7)
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 22.3	$ 0.0	$ 0.0	$ 22.3	$ 0.0