CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDER'S EQUITY (Unaudited) (USD $) In Millions, unless otherwise specified	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2011	$ 2,494.0	$ 100.9	$ 910.5	$ 1,485.1	$ (2.5)
Changes in Stockholders' Equity
Net income	252.7	0	0	252.7	0
Other comprehensive income (loss), net of tax	1.0	0	0	0	1.0
Dividends declared on common stock	(75.0)	0	0	(75.0)	0
Stock-based compensation	1.8	0	1.8	0	0
Balance at Sep. 30, 2012	2,674.5	100.9	912.3	1,662.8	(1.5)
Balance at Dec. 31, 2012	2,703.1	100.9	913.1	1,690.4	(1.3)
Changes in Stockholders' Equity
Net income	263.5	0	0	263.5	0
Other comprehensive income (loss), net of tax	1.0	0	0	0	1.0
Dividends declared on common stock	(120.0)	0	0	(120.0)	0
Stock-based compensation	1.7	0	1.7	0	0
Balance at Sep. 30, 2013	$ 2,849.3	$ 100.9	$ 914.8	$ 1,833.9	$ (0.3)