Retirement Plans (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2010	Dec. 31, 2011
Components of net periodic benefit cost [Abstract]
Capitalization of pension and OPEB expense related to engineering and plant construction		$ 4,200,000	$ 3,100,000
Range of full year estimated pension and other postretirement benefit expenses - lower		75,000,000
Range of full year estimated pension and other postretirement benefit expenses - upper		85,000,000
Full year estimated pension and other postretirement benefit expenses			58,300,000
Expected cash contributions		60,000,000
Number of pension plans that were merged with the pension plan		12
Fair value of plan assets for Acquired Business as of acquisition date	106,900,000	18,700,000		438,800,000
Receivable of the Plan that was transferred related to the Transaction		47,500,000
Plan assets		1,315,500,000			1,258,000,000
Increase (decrease) in plan assets		57,500,000
Percentage Of Increase In Pension Plan Assets		5.00%
Positive investment returns related to increase in plan assets		83,500,000
Benefit payments		26,000,000
Pension Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost		10,492,000	9,615,000
Interest cost on projected benefit obligation		19,658,000	21,172,000
Expected return on plan assets		(24,089,000)	(25,589,000)
Amortization of prior service cost/(credit)		(50,000)	(50,000)
Amortization of unrecognized loss		7,787,000	4,435,000
Net periodic benefit cost		13,798,000	9,583,000
Employer contributions		0
OPEB [Member]
Components of net periodic benefit cost [Abstract]
Service cost		2,555,000	4,228,000
Interest cost on projected benefit obligation		4,460,000	6,328,000
Expected return on plan assets		(55,000)	(82,000)
Amortization of prior service cost/(credit)		(2,503,000)	(2,550,000)
Amortization of unrecognized loss		1,911,000	1,499,000
Net periodic benefit cost		$ 6,368,000	$ 9,423,000