CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 130.3	$ 153.1
Cash flow hedges
Reclassification of net losses (gains) to net income, net of tax of $0.1 million and $0.9 million, respectively	0.1	(0.6)
Defined benefit plans
Pension and other postretirement benefit costs arising during period, net of tax of an insignificant amount for all periods presented	0	(0.1)
Amortization of pension and other postretirement benefit costs included in net periodic benefit cost, net of tax of $0.5 million and $0.3 million, respectively	0.7	0.3
Defined benefit plans, net	0.7	0.2
Other comprehensive income (loss), net of tax	0.8	(0.4)
Comprehensive income	131.1	152.7
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Noncontrolling interest in subsidiaries	0	0.1
Comprehensive income attributed to common shareholders	$ 130.3	$ 152.0