CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 153.1	$ 188.3
Cash flow hedges
Unrealized net gains arising during period, net of tax of $ – million and $ – million, respectively	0	0.1
Reclassification of net (gains) losses to net income, net of tax of $0.9 million and $0.6 million, respectively	(0.6)	0.9
Cash flow hedges, net	(0.6)	1.0
Defined benefit plans
Pension and other postretirement benefit costs arising during period, net of tax of $(0.1) million and $ – million, respectively	(0.1)	0
Amortization of pension and other postretirement benefit costs included in net periodic benefit cost, net of tax of $0.3 million and $0.4 million, respectively	0.3	0.6
Defined benefit plans, net	0.2	0.6
Other comprehensive income (loss), net of tax	(0.4)	1.6
Comprehensive income	152.7	189.9
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Noncontrolling interest in subsidiaries	0.1	0
Comprehensive income attributed to common shareholders	$ 152.0	$ 189.1