Fresh start accounting (Balance Sheet, Additional Information) (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 21, 2017	Nov. 30, 2017	Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016	Mar. 31, 2017
Fresh-Start Adjustment [Line Items]
Payment in full to extinguish Prior Credit Facility	$ (444,440)	$ (10,900)
Liabilities subject to compromise	0		$ 0
Fair value of equity issued in settlement of Prior Senior Notes and certain general unsecured creditors	(948,944)		(948,944)
Gain on settlement of liabilities subject to compromise				$ 0	$ (48)
Write-off unamortized issuance costs associated with Prior Credit Facility				0	0
Long-term debt and capital leases, classified as current	4,632		4,632	3,273	3,479
Long-term debt and capital leases, less current maturities	291,429		291,429	141,386	303,992
Cancellation of predecessor equity - par value	450		450
Cancellation of predecessor equity - paid in capital	948,613		948,613	961,200	974,616
Write-off unamortized issuance costs associated with Prior Credit Facility				$ 0	$ 0
Predecessor
Fresh-Start Adjustment [Line Items]
Prior Senior Notes and associated accrued interest	1,267,410		1,267,410
Accounts payable and accrued liabilities	6,687		6,687
Accrued payroll and benefits payable	3,949		3,949
Revenue distribution payable	3,050		3,050
Liabilities subject to compromise	1,281,096		1,281,096
Amounts settled in cash, reinstated or otherwise reserved at emergence	(10,089)		(10,089)
Fair value of equity issued in settlement of Prior Senior Notes and certain general unsecured creditors	(898,914)		(898,914)
Gain on settlement of liabilities subject to compromise	372,093		372,093			$ 0
Write-off unamortized issuance costs associated with Prior Credit Facility			1,687			0
Long-term debt and capital leases, classified as current	468,814		468,814
Long-term debt and capital leases, less current maturities	0		0
Cancellation of predecessor equity - par value	14		14
Cancellation of predecessor equity - paid in capital	$ 425,425		425,425
Write-off unamortized issuance costs associated with Prior Credit Facility			(1,687)			$ 0
Common stock outstanding
Fresh-Start Adjustment [Line Items]
Common stock shares issued (in shares)	44,982,142
Reorganization Adjustments
Fresh-Start Adjustment [Line Items]
Cash proceeds from rights offering	$ 50,031
Payment in full to extinguish Prior Credit Facility	(444,440)
Payment of accrued interest on Prior Credit Facility	(2,095)
Payment of previously accrued creditor-related professional fees	(6,954)
Net cash used	(135,333)
Total issuance costs on Exit Credit Facility	2,410		2,410
Liabilities subject to compromise	(1,281,096)		(1,281,096)
Gain on settlement of liabilities subject to compromise	372,093
Reclassification from current to noncurrent, based on scheduled repayment, of debt no longer in default	(22,612)		(22,612)
Write-off unamortized issuance costs associated with Prior Credit Facility	1,687
Long-term debt and capital leases, classified as current	(464,182)		(464,182)
Reclassification from current to noncurrent, based on scheduled repayment, of debt no longer in default	22,612		22,612
Long-term debt and capital leases, less current maturities	291,429		291,429
Cancellation of predecessor equity - par value	450		450
Cancellation of predecessor equity - paid in capital	948,613		948,613
Issuance of successor common stock in settlement of claims	898,914		898,914
Issuance of successor common stock under rights offering	50,031		50,031
Issuance of warrants	118		118
Net impact to common stock-par and additional paid in capital	523,624		523,624
Cancellation of predecessor equity	425,438
Write-off unamortized issuance costs associated with Prior Credit Facility	(1,687)
Net impact to retained earnings	795,844		795,844
Reorganization Adjustments | Predecessor
Fresh-Start Adjustment [Line Items]
Cancellation of predecessor equity - par value	(14)		(14)
Cancellation of predecessor equity - paid in capital	(425,425)		(425,425)
Reorganization Adjustments | Common stock outstanding
Fresh-Start Adjustment [Line Items]
Warrants issued for purchase of common stock (in shares)							140,023
Reorganization Adjustments | Exit Credit Facility
Fresh-Start Adjustment [Line Items]
Total issuance costs on Exit Credit Facility	535		535
Reorganization Adjustments | Exit Term Loan
Fresh-Start Adjustment [Line Items]
Cash proceeds	150,000
Fees paid to lender	(750)
Total issuance costs on Exit Credit Facility	750		750
Establishment of Exit Term Loan - current portion	1,183		1,183
Reorganization Adjustments | Exit Revolver
Fresh-Start Adjustment [Line Items]
Cash proceeds	120,000
Fees paid to lender	(1,125)
Total issuance costs on Exit Credit Facility	1,125		1,125
Reorganization Adjustments | Existing Credit Facility | Exit Term Loan
Fresh-Start Adjustment [Line Items]
Origination of the Exit Term Loan, net of current portion	148,817		148,817
Reorganization Adjustments | Existing Credit Facility | Exit Revolver
Fresh-Start Adjustment [Line Items]
New credit facility	$ 120,000		$ 120,000