CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement of Other Comprehensive Income [Abstract]
Net income	$ 354.8	$ 284.3	$ 230.5
Cash flow hedges
Unrealized net gains (losses) arising during period, net of tax of $0.1 million, $(0.1) million, and $0.4 million, respectively	0.7	(0.2)	1.5
Reclassification of net losses to net income, net of tax of $3.6 million, $2.0 million, and $4.4 million, respectively	1.4	6.5	7.4
Cash flow hedges, net	2.1	6.3	8.9
Defined benefit plans
Pension and other postretirement benefit adjustments arising during period, net of tax of $8.9 million, $(4.4) million, and $(5.7) million, respectively	13.2	(6.1)	(7.5)
Amortization of pension and other postretirement benefit costs included in net periodic benefit cost, net of tax of $1.7 million, $1.0 million, and $0.6 million, respectively	2.4	1.4	0.8
Defined benefit plans, net	15.6	(4.7)	(6.7)
Other comprehensive income, net of tax	17.7	1.6	2.2
Comprehensive income	372.5	285.9	232.7
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0.1	0.2	0
Comprehensive income attributed to common shareholders	$ 369.5	$ 283.0	$ 229.6