Retirement Plans (Details) (USD $)	6 Months Ended		12 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Pension Benefits [Member]	Jun. 30, 2012 Pension Benefits [Member]	Jun. 30, 2013 Pension Benefits [Member]	Jun. 30, 2012 Pension Benefits [Member]	Jun. 30, 2013 Postretirement Benefits Other Than Pensions (OPEB) [Member]	Jun. 30, 2012 Postretirement Benefits Other Than Pensions (OPEB) [Member]	Jun. 30, 2013 Postretirement Benefits Other Than Pensions (OPEB) [Member]	Jun. 30, 2012 Postretirement Benefits Other Than Pensions (OPEB) [Member]
Components of net periodic benefit cost [Abstract]
Service cost				$ 12,834,000	$ 10,492,000	$ 25,668,000	$ 20,984,000	$ 3,179,000	$ 2,555,000	$ 6,358,000	$ 5,110,000
Interest cost on projected benefit obligation				18,880,000	19,658,000	37,760,000	39,316,000	4,440,000	4,460,000	8,880,000	8,920,000
Expected return on plan assets				(24,590,000)	(24,089,000)	(49,180,000)	(48,177,000)	(43,000)	(55,000)	(86,000)	(110,000)
Amortization of prior service cost/(credit)				2,000	(50,000)	4,000	(100,000)	(1,525,000)	(2,503,000)	(3,050,000)	(5,006,000)
Amortization of unrecognized loss				10,072,000	7,787,000	20,144,000	15,573,000	2,228,000	1,911,000	4,456,000	3,822,000
Net periodic benefit cost				17,198,000	13,798,000	34,396,000	27,596,000	8,279,000	6,368,000	16,558,000	12,736,000
Capitalization of pension and OPEB expense related to engineering and plant construction	10,000,000	8,000,000
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	8,608,000	27,853,000	81,600,000
Estimated contributions to pension plan in current fiscal year	60,000,000
Contributions to pension plan	25,500,000
Plan assets	1,165,700,000		1,253,600,000
Decrease in plan assets	87,900,000
Percentage of decrease in pension plan assets	7.00%
Benefit payments	142,900,000
Positive investment returns	$ 29,500,000