CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 84.0	$ 38.8	$ 245.1	$ 222.4
Cash flow hedges
Unrealized net gains arising during period, net of tax of an insignificant amount for all periods presented	0	0	0	0.7
Reclassification of net losses (gains) to net income, net of tax of $0.2 million, $0.2 million, $1.1 million, and $1.7 million, respectively	0.1	0.3	(0.3)	2.7
Cash flow hedges, net	0.1	0.3	(0.3)	3.4
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	(0.1)	0
Amortization of pension and other postretirement benefit costs included in net periodic benefit cost, net of tax of $0.2 million, $0.4 million, $0.7 million, and $1.2 million, respectively	0.4	0.6	1.2	1.8
Defined benefit plans, net	0.4	0.6	1.1	1.8
Other comprehensive income, net of tax	0.5	0.9	0.8	5.2
Comprehensive income	84.5	39.7	245.9	227.6
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries	0	0	0.1	0.1
Comprehensive income attributed to common shareholders	$ 83.8	$ 39.0	$ 243.7	$ 225.4