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	85.5	0	0	85.5	0
Other comprehensive income (loss), net of tax	0.5	0	0	0	0.5
Dividends declared on common stock	0
Stock-based compensation	1.2	0	1.2	0	0
Balance at Jun. 30, 2012	2,581.2	100.9	911.7	1,570.6	(2.0)
Balance at Dec. 31, 2012	2,703.1	100.9	913.1	1,690.4	(1.3)
Changes in Stockholders' Equity
Net income	92.0	0	0	92.0	0
Other comprehensive income (loss), net of tax	0.7	0	0	0	0.7
Dividends declared on common stock	(80.0)	0	0	(80.0)	0
Stock-based compensation	1.1	0	1.1	0	0
Balance at Jun. 30, 2013	$ 2,716.9	$ 100.9	$ 914.2	$ 1,702.4	$ (0.6)