CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Other Comprehensive Income [Abstract]
Net income	$ 38.8	$ 66.3	$ 222.4	$ 215.6
Cash flow hedges
Unrealized net gains (losses) arising during period, net of tax of $ – million, $0.1 million, $ – million, and $(0.1) million, respectively	0	0.1	0.7	(0.1)
Reclassification of net losses to net income, net of tax of $0.2 million, $1.0 million, $1.7 million, and $2.6 million, respectively	0.3	1.6	2.7	4.1
Cash flow hedges, net	0.3	1.7	3.4	4.0
Defined benefit plans
Amortization of pension and other postretirement benefit costs included in net periodic benefit cost, net of tax of $0.4 million, $0.2 million, $1.2 million, and $0.7 million, respectively	0.6	0.4	1.8	1.1
Other comprehensive income, net of tax	0.9	2.1	5.2	5.1
Comprehensive income	39.7	68.4	227.6	220.7
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries	0	0.1	0.1	0.1
Comprehensive income attributed to common shareholders	$ 39.0	$ 67.8	$ 225.4	$ 218.5