Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

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No definition available.

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The cash inflow associated with the amount received from entity's raising of capital via private rather than public placement.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Long-Term Debt (Details) - USD ($)	9 Months Ended	12 Months Ended
Long-Term Debt (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2016
Debt Instrument [Line Items]
Proceeds from Issuance of Private Placement	$ 500,000,000
Long-term Debt		$ 2,724,672,000	$ 2,705,718,000
Unamortized Debt Issuance Expense		27,699,000	33,017,000
Less amounts due in one year		(1,677,000)	(1,819,000)
Long-term Debt, Excluding Current Maturities		2,722,995,000	2,703,899,000
Debt and Capital Lease Obligations		10,323,000	7,590,000
Gains (Losses) on Extinguishment of Debt		0	(67,751,000)	$ (95,267,000)
Senior Secured Notes due April 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt		1,854,731,000
Senior Notes due April 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt		494,344,000	493,439,000
Long-term Debt, Percentage Bearing Fixed Interest, Amount		0.0800
Debt Instrument, Unamortized Discount		7,240,000
Senior Notes 8.25 Percent Due April 2020 Amount Purchased [Member]
Debt Instrument [Line Items]
Long-term Debt		2,508,000			$ 937,822,000
Gains (Losses) on Extinguishment of Debt		(67,734,000)
Senior notes due April 2020 at 8.25%
Debt Instrument [Line Items]
Long-term Debt		74,470,000	74,470,000
Long-term Debt, Percentage Bearing Fixed Interest, Amount		0.0825
Gains (Losses) on Extinguishment of Debt		(17,000)
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		400,000,000
Long-term Debt		201,000,000	185,000,000
Senior Secured Notes Due March 2021 Par Value Amount Purchased [Member]
Debt Instrument [Line Items]
Long-term Debt		213,000			$ 229,176,000
Senior notes due March 2021 at 6.375%
Debt Instrument [Line Items]
Long-term Debt		20,611,000	20,611,000
Long-term Debt, Percentage Bearing Fixed Interest, Amount		0.06375
Senior Secured Notes Due April 2022 Par Value [Member]
Debt Instrument [Line Items]
Long-term Debt			1,855,617,000
Baltimore Port Facility revenue bonds in series due September 2025 at 5.75%
Debt Instrument [Line Items]
Long-term Debt		102,865,000	102,865,000
Advance royalty commitments
Debt Instrument [Line Items]
Long-term Debt		2,678,000	3,964,000
Note due through December 2012 at 6.10%
Debt Instrument [Line Items]
Long-term Debt		$ 1,672,000	$ 2,769,000