Fresh start accounting (Balance Sheet, Additional Information) (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 21, 2017	Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2017
Fresh-Start Adjustment [Line Items]
Fees paid to lender		$ 0	$ 0	$ (1,624)	$ 0
Prior Senior Notes including 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	(948,944)	(948,944)
Gain on settlement of liabilities subject to compromise	372,093	372,093	0	0	(48)
Write-off unamortized issuance costs associated with Prior Credit Facility		1,687	0	0	0
Long-term debt and capital leases, classified as current	4,632	4,632		594	12,371
Long-term debt and capital leases, less current maturities	291,429	291,429		421,392	295,100
Cancellation of predecessor equity - par value	450	450		485	467
Cancellation of predecessor equity - paid in capital	$ 948,613	948,613		977,174	974,616
Write-off unamortized issuance costs associated with Prior Credit Facility		(1,687)	$ 0	$ 0	$ 0
Common Stock
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
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 | Common Stock
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]
Origination of the Exit Revolver	120,000	120,000
Predecessor
Fresh-Start Adjustment [Line Items]
Liabilities subject to compromise	1,281,096	1,281,096
Fair value of equity issued in settlement of Prior Senior Notes and certain general unsecured creditors	(898,914)	(898,914)
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
Predecessor | Reorganization Adjustments
Fresh-Start Adjustment [Line Items]
Cancellation of predecessor equity - par value	(14)	(14)
Cancellation of predecessor equity - paid in capital	(425,425)	(425,425)
Successor | Reorganization Adjustments
Fresh-Start Adjustment [Line Items]
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