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, 2014	$ 3,244.4	$ 2.0	$ 1,085.6	$ 2,198.2	$ (41.4)
Changes in Stockholders' Equity
Net income	241.9	0.0	0.0	241.9	0.0
Other comprehensive income, net of tax	5.3	0.0	0.0	0.0	5.3
Dividends declared on common stock	(154.8)	0.0	0.0	(154.8)	0.0
Issuance of common stock	7.2	0.0	7.2	0.0	0.0
Stock-based compensation	9.4	0.0	9.4	0.0	0.0
Balance at Sep. 30, 2015	3,353.4	2.0	1,102.2	2,285.3	(36.1)
Balance at Dec. 31, 2015	3,326.0	2.0	1,099.3	2,259.8	(35.1)
Changes in Stockholders' Equity
Net income	280.3	0.0	0.0	280.3	0.0
Other comprehensive income, net of tax	6.1	0.0	0.0	0.0	6.1
Dividends declared on common stock	(170.3)	0.0	0.0	(170.3)	0.0
Stock-based compensation	3.1	0.0	3.1	0.0	0.0
Balance at Sep. 30, 2016	$ 3,445.2	$ 2.0	$ 1,102.4	$ 2,369.8	$ (29.0)