Debt - Additional Information (Details)								3 Months Ended		9 Months Ended
	Mar. 01, 2017 USD ($)	Jun. 14, 2016 USD ($)	Jun. 13, 2016 USD ($)	Jun. 10, 2016 $ / shares	May 27, 2016	May 13, 2016 USD ($)	Mar. 06, 2012 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) instalment $ / shares	Sep. 30, 2015 USD ($)	Nov. 07, 2016 USD ($)	Apr. 13, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 24, 2014 USD ($)	Nov. 27, 2013 USD ($)	Nov. 08, 2012 USD ($)
Debt Instrument [Line Items]
Current portion of long-term debt								$ 292,795,000		$ 292,795,000				$ 0
Fair value of amount outstanding								$ 341,500,000		$ 341,500,000			$ 457,000,000
Repayments of lines of credit		$ 56,800,000
Consolidated funded debt to consolidated EBITDA ratio		3.75
Long-term debt, weighted average interest rate								3.10%		3.10%
Consolidated EBITDA To consolidated net interest expense								2.5		2.5
Stock split, conversion ratio				0.1	0.1
Long-term debt								$ 1,415,700,000		$ 1,415,700,000				1,061,000,000
Interest expense related to contractual interest coupon of convertible notes								1,300,000	$ 1,300,000	3,900,000	$ 3,900,000
Accrued interest								22,917,000		$ 22,917,000				$ 9,022,000
Minimum
Debt Instrument [Line Items]
Percentage of borrowing base utilization, percent										1.50%
Maximum
Debt Instrument [Line Items]
Percentage of borrowing base utilization, percent										2.50%
Subsequent Event
Debt Instrument [Line Items]
Fair value of amount outstanding												$ 341,500,000
Quarter ended June 30, 2016
Debt Instrument [Line Items]
Consolidated funded debt to consolidated EBITDA ratio		5.25
Quarter ending September 30, 2016
Debt Instrument [Line Items]
Consolidated funded debt to consolidated EBITDA ratio		6.50
Quarter ending December 31, 2016
Debt Instrument [Line Items]
Consolidated funded debt to consolidated EBITDA ratio		9.50
Thereafter
Debt Instrument [Line Items]
Consolidated funded debt to consolidated EBITDA ratio		3.75
1 3⁄4% Senior Convertible Notes due 2017
Debt Instrument [Line Items]
Current portion of long-term debt								$ 292,400,000		$ 292,400,000
Aggregate principal amount of senior subordinated notes							$ 300,000,000
Initial conversion rate of common stock				0.0023449			0.0234449			1
Initial conversion price of convertible note 2017 (in usd per share) | $ / shares				$ 426.50			$ 42.65
Closing share price (in usd per share) | $ / shares								$ 11.88		$ 11.88
Payment for call option							$ 70,800,000
Anti-dilution adjustments for purchases of call option (in shares) | shares							703,347
Strike price per share (in usd per share) | $ / shares							$ 559.10
Proceeds from sale of warrants							$ 40,100,000
Interest expense related to amortization of discount								$ 4,100,000	3,800,000	$ 12,000,000	11,100,000
Amortization of deferred financing costs								400,000	$ 400,000	1,100,000	$ 1,100,000
1 3⁄4% Senior Convertible Notes due 2017 | Scenario, Forecast
Debt Instrument [Line Items]
Cash share holders receive for each dollar In principle	$ 1,000
4.20% Building Loan
Debt Instrument [Line Items]
Current portion of long-term debt								400,000		400,000
Revolving credit facility
Debt Instrument [Line Items]
Current portion of long-term debt													175,300,000
Redetermined base borrowing and credit facility															$ 900,000,000
Borrowing base		$ 360,000,000											300,000,000		$ 500,000,000
Outstanding borrowing under bank credit facility								12,500,000		$ 12,500,000			$ 18,300,000
Period in which outstanding amount has to be repaid to cure deficiency										10 days
Period in which bank has to add new properties to borrowing base and has to grant mortgage to banks										30 days
Number of monthly installments | instalment										6
Repayments of lines of credit			$ 29,200,000			$ 29,200,000
Line of credit facility, minimum liquidity requirement		125,000,000.0
Line of credit facility, limitations on capital expenditures		60,000,000
Line of credit facility, amount of cash to which anti-hoarding cash provisions apply		50,000,000
Initial bank and availability under facility								6,000,000		$ 6,000,000
Oil and gas reserve as proportion of discounted present value of future net cash flow, for mortgage, percent										86.00%
Revolving credit facility | Subsequent Event
Debt Instrument [Line Items]
Outstanding borrowing under bank credit facility												12,500,000
Initial bank and availability under facility												$ 6,000,000
Revolving credit facility | Uncompleted Wells
Debt Instrument [Line Items]
Line of credit facility, limitations on capital expenditures		$ 25,000,000
Senior Notes Due 2022
Debt Instrument [Line Items]
Aggregate principal amount of senior subordinated notes								775,000,000		$ 775,000,000						$ 475,000,000	$ 300,000,000
Accrued interest								$ 29,200,000		$ 29,200,000
Grace period to make interest payment										30 days
Senior Notes Due 2022 | Minimum
Debt Instrument [Line Items]
Percentage of minimum principal amount before an event of default										25.00%