CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY (USD $) In Millions, unless otherwise specified	Total	Shareholder's Equity	Common Stock	Additional Paid-in Capital	Retained Earnings
Balance at Dec. 31, 2010		$ 1,147.2	$ 95.6	$ 626.7	$ 424.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholder	125.9	122.8	0	0	122.8
Equity contribution from parent		20.0	0	20.0	0
Return of capital to parent		(89.3)	0	(89.3)	0
Dividends to parent		(102.5)	0	0	(102.5)
Other		3.9	0	4.5	(0.6)
Balance at Dec. 31, 2011		1,102.1	95.6	561.9	444.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholder	134.8	131.7	0	0	131.7
Equity contribution from parent		40.0	0	40.0	0
Return of capital to parent		(50.0)	0	(50.0)	0
Dividends to parent		(105.5)	0	0	(105.5)
Other		3.2	0	3.5	(0.3)
Balance at Dec. 31, 2012		1,121.5	95.6	555.4	470.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholder	137.9	134.8	0	0	134.8
Equity contribution from parent		200.0	0	200.0	0
Return of capital to parent		(35.0)	0	(35.0)	0
Dividends to parent		(108.6)	0	0	(108.6)
Other		2.7	0	3.1	(0.4)
Balance at Dec. 31, 2013		$ 1,315.4	$ 95.6	$ 723.5	$ 496.3