Long-Term Debt - Outstanding Debt (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal	$ 625,000,000	$ 626,859,000
Debt Issuance Costs	(7,613,000)	(9,115,000)
Carrying Amount	617,387,000	617,744,000
Debt Instrument, Fair Value Disclosure	594,400,000	661,400,000
Amended Credit Facility [Member]
Debt Instrument [Line Items]
Principal	0	0
Debt Issuance Costs	0	0
Carrying Amount	0	0
Debt Instrument, Fair Value Disclosure	$ 0
7.625% Senior Notes [Member]
Debt Instrument [Line Items]
Principal			$ 315,300,000
Debt, stated interest rate	7.625%
7.0% Senior Notes [Member]
Debt Instrument [Line Items]
Principal	$ 350,000,000.0	350,000,000
Debt Issuance Costs	(3,210,000)	(4,033,000)
Carrying Amount	$ 346,790,000	345,967,000
Debt, stated interest rate	7.00%
Debt Instrument, Fair Value Disclosure	$ 329,700,000	356,100,000
Eight Point Seven Five Percentage Senior Notes [Member]
Debt Instrument [Line Items]
Principal	275,000,000	275,000,000
Debt Issuance Costs	(4,403,000)	(5,080,000)
Carrying Amount	$ 270,597,000	269,920,000
Debt, stated interest rate	8.75%
Debt Instrument, Fair Value Disclosure	$ 264,700,000	305,300,000
Lease Financing Obligation [Member]
Debt Instrument [Line Items]
Principal	1,859,000	2,328,000
Debt Issuance Costs	0	(2,000)
Carrying Amount	1,859,000	2,326,000
Debt Instrument, Fair Value Disclosure	1,800,000	2,100,000
Total Debt [Member]
Debt Instrument [Line Items]
Principal	626,859,000	627,328,000
Debt Issuance Costs	(7,613,000)	(9,115,000)
Carrying Amount	619,246,000	618,213,000
Current Portion of Long-Term Debt [Member]
Debt Instrument [Line Items]
Principal	1,859,000	469,000
Debt Issuance Costs	0	0
Carrying Amount	$ 1,859,000	$ 469,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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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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