CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDER'S EQUITY (USD $) In Millions, unless otherwise specified	Total	Total Common Shareholder's Equity	Common Stock	Additional Paid in Capital	Retained Earnings
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	131.7	131.7	0	0	131.7
Equity contribution from parent	40.0	40.0	0	40.0	0
Return of capital to parent	50.0	(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	134.8	134.8	0	0	134.8
Equity contribution from parent	200.0	200.0	0	200.0	0
Return of capital to parent	35.0	(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	1,315.4	95.6	723.5	496.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributed to common shareholder	137.6	137.6	0	0	137.6
Equity contribution from parent	55.0	55.0	0	55.0	0
Return of capital to parent	0
Dividends to parent		(111.8)	0	0	(111.8)
Other		3.2	0	3.5	(0.3)
Balance at Dec. 31, 2014	$ 1,399.4	$ 1,399.4	$ 95.6	$ 782.0	$ 521.8