Common Equity (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Calculation of Basic and Diluted Earnings Per Share

	Year Ended December 31,
(in millions, except per share data)	2017		2016		2015
Basic earnings per common share:
Net income available to common shareholders	$	389.9	$	156.9	$	140.9
Weighted average common shares issued and outstanding	171.1		156.6		155.3
Basic earnings per common share	$	2.28	$	1.00	$	0.91
Diluted earnings per common share:
Net income applicable to diluted earnings per common share	$	389.9	$	156.9	$	140.9
Dilutive potential common shares:
Restricted share units	0.2		0.2		0.3
Dilutive potential common shares	0.2		0.2		0.3
Total diluted average common shares issued and outstanding	171.3		156.8		155.6
Diluted earnings per common share	$	2.28	$	1.00	$	0.91