Employee Benefits (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Aug. 03, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution to defined benefit pension plan					$ 4.8
Defined Contribution Plans [Abstract]
Costs related to the defined contribution plans	1.9	1.8	3.8	3.7
Retirement Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1.0	0.9	2.0	1.8
Interest cost	1.6	1.7	3.2	3.4
Expected return on plan assets	(2.0)	(1.7)	(4.0)	(3.4)
Amortization of prior service credit	0	0	0	0
Amortization of unrecognized net loss	0.3	0.3	0.6	0.7
Regulatory asset decrease	0	0	0	0
Net periodic benefit cost	0.9	1.2	1.8	2.5
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.6	0.7	1.2	1.4
Expected return on plan assets	(0.8)	(1.0)	(1.6)	(1.9)
Amortization of prior service credit	(1.9)	(1.9)	(3.8)	(3.8)
Amortization of unrecognized net loss	0.1	0.2	0.3	0.4
Regulatory asset decrease	1.3	1.3	2.7	2.7
Net periodic benefit cost	$ (0.6)	$ (0.6)	$ (1.0)	$ (1.0)