Investments in equity affiliates and joint ventures.

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Common and/or restricted stock relinquished during the period. Recorded using the cost method.

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Supplemental Guarantor Non Guarantor Financial Information Textuals Abstract.

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Transactions with affiliates, net.

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Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).

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The increase (decrease) during the reporting period in cash and cash equivalents. While for technical reasons this element has no balance attribute, the default assumption is a debit balance consistent with its label.

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This element represents cash provided by or used in the operating activities of the entity's discontinued operations during the period. This element is only used by those entities that separately report cash flows attributable to discontinued operations. If using this element, it is an indication that the cash flows of the entity which are detailed in reconciling to cash provided by or used in operating activities reflect only cash flows attributable to continuing operations.

+ References

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- Definition

Interest rate stated in the contractual debt agreement.

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Reductions in the entity's income taxes that arise when compensation cost (from non-qualified share-based compensation) recognized on the entity's tax return exceeds compensation cost from equity-based compensation recognized in financial statements. This element represents the cash inflow reported in the enterprise's financing activities.

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- Definition

The net cash inflow or outflow from financing activity for the period.

+ References

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- Definition

The net cash inflow or outflow from investing activity.

+ References

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The net cash from (used in) all of the entity's operating activities, including those of discontinued operations, of the reporting entity. Operating activities generally involve producing and delivering goods and providing services. Operating activity cash flows include transactions, adjustments, and changes in value that are not defined as investing or financing activities. While for technical reasons this element has no balance attribute, the default assumption is a debit balance consistent with its label.

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The net cash from (used in) the entity's continuing operations. This element specifically EXCLUDES the cash flows derived by the entity from its discontinued operations, if any. This element is only to be used when the entity reports its cash flows attributable to discontinued operations separately from the cash flow provided by or used in operating activities. While for technical reasons this element has no balance attribute, the default assumption is a debit balance consistent with its label.

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- Definition

The net cash outflow or inflow from other investing activities. This element is used when there is not a more specific and appropriate element in the taxonomy.

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- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The cash outflow from the distribution of an entity's earnings in the form of dividends to common shareholders.

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- Definition

The cash outflow associated with the investment in or advances to an entity in which the reporting entity shares control of the entity with another party or group.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The cash outflow from purchases of trading, available-for-sale securities and held-to-maturity securities.

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- Definition

The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

+ References

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- Definition

The cash outflow for securities or other assets acquired, which qualify for treatment as an investing activity and are to be liquidated, if necessary, within the current operating cycle. Includes cash flows from securities classified as trading securities that were acquired for reasons other than sale in the short-term.

+ References

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- Definition

The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer).

+ References

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Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Nov. 30, 2026	Sep. 30, 2020	Dec. 31, 2016	Nov. 30, 2016	Mar. 31, 2016	Dec. 31, 2014	Mar. 31, 2013	Apr. 15, 2011
Cash Flows From Operating Activities
Net cash provided by (used in) continuing operations	$ 618.2	$ 474.3
Net cash used in discontinued operations	(2.6)	(9.9)
Net cash provided by (used in) operating activities	615.6	464.4
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(354.8)	(187.5)
Investment in Prairie State Energy Campus	(21.5)	(30.5)
Proceeds from disposal of assets	9.6	6.1
Investments in equity affiliates and joint ventures	(2.4)	(17.4)
Proceeds from sale of debt securities	21.0
Purchases of debt securities	(14.6)
Purchases of short-term investments	(100.0)
Maturity of short-term investments	25.0
Other, net	(2.1)	(4.4)
Net cash used in investing activities	(439.8)	(233.7)
Cash Flows From Financing Activities
Proceeds from long-term debt	1.4	500.0
Payments of long-term debt	(238.2)	(495.7)
Dividends paid	(46.8)	(37.6)
Repurchase of employee common stock relinquished for tax withholding	(15.4)	(8.0)
Payment of debt issuance costs		(21.9)
Excess tax benefits related to share-based compensation	5.8
Proceeds from stock options exercised	4.4	2.4
Other, net	(5.3)	(1.7)
Net cash provided by (used in) financing activities	(294.1)	(62.5)
Net change in cash and cash equivalents	(118.3)	168.2
Cash and cash equivalents at beginning of period	1,295.2	988.8
Cash and cash equivalents at end of period	1,176.9	1,157.0
Supplemental Guarantor/Non-Guarantor Financial Information (Textuals)
Interest rates on Senior Notes			7.875%	6.50%	6.84%	7.375%	5.875%	6.34%	6.875%	5.875%
Parent Company [Member]
Cash Flows From Operating Activities
Net cash provided by (used in) continuing operations	43.8	(98.1)
Net cash used in discontinued operations	(1.4)	(8.8)
Net cash provided by (used in) operating activities	42.4	(106.9)
Cash Flows From Investing Activities
Purchases of short-term investments	(75.0)
Maturity of short-term investments	25.0
Net cash used in investing activities	(50.0)
Cash Flows From Financing Activities
Proceeds from long-term debt		500.0
Payments of long-term debt	(230.6)	(490.3)
Dividends paid	(46.8)	(37.6)
Repurchase of employee common stock relinquished for tax withholding	(15.4)	(8.0)
Payment of debt issuance costs		(21.9)
Excess tax benefits related to share-based compensation	5.8
Proceeds from stock options exercised	4.4	2.4
Other, net	3.0	2.8
Transactions with affiliates, net	102.7	240.1
Net cash provided by (used in) financing activities	(176.9)	187.5
Net change in cash and cash equivalents	(184.5)	80.6
Cash and cash equivalents at beginning of period	903.8	368.4
Cash and cash equivalents at end of period	719.3	449.0
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided by (used in) continuing operations	417.7	454.1
Net cash used in discontinued operations	(1.2)	(1.1)
Net cash provided by (used in) operating activities	416.5	453.0
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(123.6)	(159.3)
Investment in Prairie State Energy Campus	(21.5)	(30.5)
Proceeds from disposal of assets	9.6	4.9
Investments in equity affiliates and joint ventures	(2.4)	(15.0)
Other, net	(1.8)	(4.3)
Net cash used in investing activities	(139.7)	(204.2)
Cash Flows From Financing Activities
Other, net		(2.4)
Transactions with affiliates, net	(281.3)	(246.4)
Net cash provided by (used in) financing activities	(281.3)	(248.8)
Net change in cash and cash equivalents	(4.5)
Cash and cash equivalents at beginning of period	5.2	0.2
Cash and cash equivalents at end of period	0.7	0.2
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided by (used in) continuing operations	156.7	118.3
Net cash provided by (used in) operating activities	156.7	118.3
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(231.2)	(28.2)
Proceeds from disposal of assets		1.2
Investments in equity affiliates and joint ventures		(2.4)
Proceeds from sale of debt securities	21.0
Purchases of debt securities	(14.6)
Purchases of short-term investments	(25.0)
Other, net	(0.3)	(0.1)
Net cash used in investing activities	(250.1)	(29.5)
Cash Flows From Financing Activities
Proceeds from long-term debt	1.4
Payments of long-term debt	(7.6)	(5.4)
Other, net	(8.3)	(2.1)
Transactions with affiliates, net	178.6	6.3
Net cash provided by (used in) financing activities	164.1	(1.2)
Net change in cash and cash equivalents	70.7	87.6
Cash and cash equivalents at beginning of period	386.2	620.2
Cash and cash equivalents at end of period	$ 456.9	$ 707.8