Maximum Potential Total of Future Payments Under Commitment Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	$ 1,642,981
Letters of Credit:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	335,949
Letters of Credit: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	198,447
Letters of Credit: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	56,994
Letters of Credit: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	80,508
Surety Bonds:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	675,369
Surety Bonds: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	204,895
Surety Bonds: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	442,698
Surety Bonds: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	27,776
Guarantees:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	631,663
Guarantees: | Gas
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	100,223
Guarantees: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	451,640
Guarantees: | Coal Segment [Member]
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	79,800
Less Than 1 Year
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	1,032,977
Less Than 1 Year | Letters of Credit:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	195,282
Less Than 1 Year | Letters of Credit: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	128,645
Less Than 1 Year | Letters of Credit: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	23,076
Less Than 1 Year | Letters of Credit: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	43,561
Less Than 1 Year | Surety Bonds:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	672,093
Less Than 1 Year | Surety Bonds: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	204,895
Less Than 1 Year | Surety Bonds: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	439,435
Less Than 1 Year | Surety Bonds: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	27,763
Less Than 1 Year | Guarantees:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	165,602
Less Than 1 Year | Guarantees: | Gas
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	54,613
Less Than 1 Year | Guarantees: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	80,237
Less Than 1 Year | Guarantees: | Coal Segment [Member]
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	30,752
1-3 Years
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	325,120
1-3 Years | Letters of Credit:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	140,667
1-3 Years | Letters of Credit: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	69,802
1-3 Years | Letters of Credit: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	33,918
1-3 Years | Letters of Credit: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	36,947
1-3 Years | Surety Bonds:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	3,275
1-3 Years | Surety Bonds: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
1-3 Years | Surety Bonds: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	3,263
1-3 Years | Surety Bonds: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	12
1-3 Years | Guarantees:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	181,178
1-3 Years | Guarantees: | Gas
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	14,988
1-3 Years | Guarantees: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	139,642
1-3 Years | Guarantees: | Coal Segment [Member]
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	26,548
3-5 Years
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	105,221
3-5 Years | Letters of Credit:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Letters of Credit: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Letters of Credit: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Letters of Credit: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Surety Bonds:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Surety Bonds: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Surety Bonds: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Surety Bonds: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Guarantees:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	105,221
3-5 Years | Guarantees: | Gas
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
3-5 Years | Guarantees: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	86,721
3-5 Years | Guarantees: | Coal Segment [Member]
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	18,500
Beyond 5 Years
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	179,663
Beyond 5 Years | Letters of Credit:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Letters of Credit: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Letters of Credit: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Letters of Credit: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Surety Bonds:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	1
Beyond 5 Years | Surety Bonds: | Employee-Related
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Surety Bonds: | Environmental
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	0
Beyond 5 Years | Surety Bonds: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	1
Beyond 5 Years | Guarantees:
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	179,662
Beyond 5 Years | Guarantees: | Gas
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	30,622
Beyond 5 Years | Guarantees: | Other Commitments
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	145,040
Beyond 5 Years | Guarantees: | Coal Segment [Member]
Loss Contingencies [Line Items]
Maximum potential total of future payments under commitments	$ 4,000