Retirement Plans (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Components of net periodic benefit cost [Abstract]
Capitalization of pension and OPEB expense related to engineering and plant construction	$ 4,900,000	$ 4,200,000
Estimated pension and other postretirement benefit expenses, minimum	100,000,000
Estimated pension and other postretirement benefit expenses, maximum	110,000,000
Pension and other postretirement benefit expenses	5,018,000	12,403,000	81,600,000
Estimated contributions to pension plan in current fiscal year	60,000,000
Contributions to pension plan	12,100,000
Plan assets	1,224,300,000		1,253,600,000
Decrease in plan assets	(29,300,000)
Percentage of decrease in pension plan assets (in hundredths)	(2.00%)
Benefit payments	89,100,000
Positive investment returns and cash contributions	59,800,000
Pension Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost	12,834,000	10,492,000
Interest cost on projected benefit obligation	18,880,000	19,658,000
Expected return on plan assets	(24,590,000)	(24,088,000)
Amortization of prior service cost/(credit)	2,000	(50,000)
Amortization of unrecognized loss	10,072,000	7,786,000
Net periodic benefit cost	17,198,000	13,798,000
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Components of net periodic benefit cost [Abstract]
Service cost	3,179,000	2,555,000
Interest cost on projected benefit obligation	4,440,000	4,460,000
Expected return on plan assets	(43,000)	(55,000)
Amortization of prior service cost/(credit)	(1,525,000)	(2,503,000)
Amortization of unrecognized loss	2,228,000	1,911,000
Net periodic benefit cost	$ 8,279,000	$ 6,368,000