Supplemental Guarantor/Non-Guarantor Financial Information (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Current assets
Cash and cash equivalents	$ 952.4	$ 799.1	$ 1,273.2	$ 1,295.2
Accounts receivable, net	748.1	922.5
Inventories	514.9	446.3
Assets from coal trading activities, net	43.3	44.6
Deferred income taxes	24.3	27.3
Other current assets	746.7	766.1
Total current assets	3,029.7	3,005.9
Property, plant, equipment and mine development, net	11,969.1	11,362.7
Investments and other assets	1,938.3	2,364.4
Total assets	16,937.1	16,733.0
Current liabilities
Current maturities of long-term debt	114.6	101.1
Payables to (receivables from) affiliates, net	0	0
Liabilities from coal trading activities, net	14.6	10.3
Deferred income taxes	0	0
Accounts payable and accrued expenses	1,502.8	1,712.3
Total current liabilities	1,632.0	1,823.7
Long-term debt, less current maturities	6,538.4	6,556.4
Deferred income taxes	590.8	554.2
Notes payable to (receivables from) affiliates, net	0	0
Other noncurrent liabilities	2,363.1	2,282.9
Total liabilities	11,124.3	11,217.2
Peabody Energy Corporation's stockholders' equity	5,726.6	5,485.1
Noncontrolling interests	86.2	30.7
Total stockholders' equity	5,812.8	5,515.8
Total liabilities and stockholders' equity	16,937.1	16,733.0
Parent Company [Member]
Current assets
Cash and cash equivalents	400.4	283.2	791.4	903.8
Accounts receivable, net	7.1	5.3
Inventories	0	0
Assets from coal trading activities, net	0	0
Deferred income taxes	0	0
Other current assets	345.1	305.1
Total current assets	752.6	593.6
Property, plant, equipment and mine development, net	0	0
Investments and other assets	10,478.4	10,300.8
Total assets	11,231.0	10,894.4
Current liabilities
Current maturities of long-term debt	75.0	62.5
Payables to (receivables from) affiliates, net	2,460.1	2,417.8
Liabilities from coal trading activities, net	0	0
Deferred income taxes	11.6	11.6
Accounts payable and accrued expenses	132.9	69.4
Total current liabilities	2,679.6	2,561.3
Long-term debt, less current maturities	6,410.5	6,428.8
Deferred income taxes	89.7	76.0
Notes payable to (receivables from) affiliates, net	(3,734.8)	(3,720.0)
Other noncurrent liabilities	59.4	63.2
Total liabilities	5,504.4	5,409.3
Peabody Energy Corporation's stockholders' equity	5,726.6	5,485.1
Noncontrolling interests	0	0
Total stockholders' equity	5,726.6	5,485.1
Total liabilities and stockholders' equity	11,231.0	10,894.4
Guarantor Subsidiaries [Member]
Current assets
Cash and cash equivalents	0.9	0.8	3.4	5.2
Accounts receivable, net	74.6	100.8
Inventories	271.4	220.0
Assets from coal trading activities, net	20.2	14.9
Deferred income taxes	46.9	48.0
Other current assets	58.1	98.8
Total current assets	472.1	483.3
Property, plant, equipment and mine development, net	4,886.4	4,908.8
Investments and other assets	270.4	199.7
Total assets	5,628.9	5,591.8
Current liabilities
Current maturities of long-term debt	0.1	0
Payables to (receivables from) affiliates, net	(2,673.6)	(2,529.1)
Liabilities from coal trading activities, net	10.8	4.2
Deferred income taxes	0	0
Accounts payable and accrued expenses	668.0	868.8
Total current liabilities	(1,994.7)	(1,656.1)
Long-term debt, less current maturities	6.7	0
Deferred income taxes	151.0	126.3
Notes payable to (receivables from) affiliates, net	(923.9)	(981.5)
Other noncurrent liabilities	1,934.6	1,892.6
Total liabilities	(826.3)	(618.7)
Peabody Energy Corporation's stockholders' equity	6,455.2	6,210.5
Noncontrolling interests	0	0
Total stockholders' equity	6,455.2	6,210.5
Total liabilities and stockholders' equity	5,628.9	5,591.8
Non-Guarantor Subsidiaries [Member]
Current assets
Cash and cash equivalents	551.1	515.1	478.4	386.2
Accounts receivable, net	666.4	816.4
Inventories	243.5	226.3
Assets from coal trading activities, net	23.1	29.7
Deferred income taxes	0	0
Other current assets	343.5	362.2
Total current assets	1,827.6	1,949.7
Property, plant, equipment and mine development, net	7,082.7	6,453.9
Investments and other assets	988.3	1,496.1
Total assets	9,898.6	9,899.7
Current liabilities
Current maturities of long-term debt	39.5	38.6
Payables to (receivables from) affiliates, net	213.5	111.3
Liabilities from coal trading activities, net	3.8	6.1
Deferred income taxes	11.0	9.1
Accounts payable and accrued expenses	701.9	774.1
Total current liabilities	969.7	939.2
Long-term debt, less current maturities	121.2	127.6
Deferred income taxes	350.1	351.9
Notes payable to (receivables from) affiliates, net	4,658.7	4,701.5
Other noncurrent liabilities	369.1	327.1
Total liabilities	6,468.8	6,447.3
Peabody Energy Corporation's stockholders' equity	3,343.6	3,421.7
Noncontrolling interests	86.2	30.7
Total stockholders' equity	3,429.8	3,452.4
Total liabilities and stockholders' equity	9,898.6	9,899.7
Eliminations [Member]
Current assets
Cash and cash equivalents	0	0
Accounts receivable, net	0	0
Inventories	0	0
Assets from coal trading activities, net	0	0
Deferred income taxes	(22.6)	(20.7)
Other current assets	0	0
Total current assets	(22.6)	(20.7)
Property, plant, equipment and mine development, net	0	0
Investments and other assets	(9,798.8)	(9,632.2)
Total assets	(9,821.4)	(9,652.9)
Current liabilities
Current maturities of long-term debt	0	0
Payables to (receivables from) affiliates, net	0	0
Liabilities from coal trading activities, net	0	0
Deferred income taxes	(22.6)	(20.7)
Accounts payable and accrued expenses	0	0
Total current liabilities	(22.6)	(20.7)
Long-term debt, less current maturities	0	0
Deferred income taxes	0	0
Notes payable to (receivables from) affiliates, net	0	0
Other noncurrent liabilities	0	0
Total liabilities	(22.6)	(20.7)
Peabody Energy Corporation's stockholders' equity	(9,798.8)	(9,632.2)
Noncontrolling interests	0	0
Total stockholders' equity	(9,798.8)	(9,632.2)
Total liabilities and stockholders' equity	$ (9,821.4)	$ (9,652.9)