Compensation Plans	6 Months Ended
Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation Plans	Note 7 - Compensation Plans Performance Share Units Under the Equity Incentive Compensation Plan The Company grants performance share units (“PSUs”) to eligible employees as part of its long-term equity compensation program. The number of shares of the Company’s common stock issued to settle PSUs ranges from 0% to 200% of the number of PSUs awarded and is determined based on certain performance criteria over a three-year measurement period. The performance criteria for PSUs are based on a combination of the Company’s annualized Total Shareholder Return (“TSR”) for the performance period and the relative performance of the Company’s TSR compared with the annualized TSR of certain peer companies for the performance period. Compensation expense for PSUs is recognized within general and administrative and exploration expense over the vesting periods of the respective awards. Total compensation expense recorded for PSUs for the three months ended June 30, 2017, and 2016, was $1.7 million and $3.0 million, respectively, and $4.2 million and $5.9 million for the six months ended June 30, 2017, and 2016, respectively. As of June 30, 2017, there was $10.6 million of total unrecognized compensation expense related to non-vested PSU awards, which is being amortized through 2019. There were no material changes to the outstanding and non-vested PSUs during the six months ended June 30, 2017. Subsequent to June 30, 2017, the Company granted 977,731 PSUs with a fair value of $15.5 million. These PSUs generally vest on the third anniversary of the date of the grant. Also, subsequent to June 30, 2017, the Company settled PSUs that were granted in 2014 with no shares issued upon settlement as the grant settled at a zero multiplier. Restricted Stock Units Under the Equity Incentive Compensation Plan The Company grants restricted stock units (“RSUs”) as part of its long-term equity compensation program. Each RSU represents a right to receive one share of the Company’s common stock upon settlement of the award at the end of the specified vesting period. Compensation expense for RSUs is recognized within general and administrative expense and exploration expense over the vesting periods of the respective awards. Total compensation expense recorded for RSUs was $2.1 million and $3.3 million for the three months ended June 30, 2017, and 2016, respectively, and $4.6 million and $6.5 million for the six months ended June 30, 2017, and 2016, respectively. As of June 30, 2017, there was $9.8 million of total unrecognized compensation expense related to non-vested RSU awards, which is being amortized through 2019. There were no material changes to the outstanding and non-vested RSUs during the six months ended June 30, 2017. Subsequent to June 30, 2017, the Company granted 1,010,298 RSUs with a fair value of $16.8 million. These RSUs generally vest one-third of the total grant on each of the next three anniversary dates of the grant. Also, subsequent to June 30, 2017, the Company settled 243,951 RSUs that related to awards granted in previous years. The Company and the majority of grant participants mutually agreed to net share settle a portion of the awards to cover income and payroll tax withholdings, as provided for in the plan document and award agreements. As a result, the Company issued 169,891 net shares of common stock upon settlement of the awards. The remaining 74,060 shares were withheld to satisfy income and payroll tax withholding obligations that occurred upon delivery of the shares underlying those RSUs. Director Shares During the second quarter of 2017, the Company issued 71,573 shares of its common stock and 8,794 RSUs to its non-employee directors, under the Company’s Equity Incentive Compensation Plan, which fully vest on December 31, 2017. The Company issued 53,473 shares of its common stock to its non-employee directors during the second quarter of 2016. Employee Stock Purchase Plan Under the Company’s Employee Stock Purchase Plan (“ESPP”), eligible employees may purchase shares of the Company’s common stock through payroll deductions of up to 15 percent of eligible compensation, without accruing in excess of $25,000 in value from purchases for each calendar year. The purchase price of the stock is 85 percent of the lower of the fair market value of the stock on either the first or last day of the purchase period. The ESPP is intended to qualify under Section 423 of the Internal Revenue Code (“IRC”). There were 123,678 and 140,853 shares issued under the ESPP during the second quarters of 2017 and 2016, respectively. The fair value of ESPP grants is measured at the date of grant using the Black-Scholes option-pricing model.

Note 7 - Compensation Plans

Performance Share Units Under the Equity Incentive Compensation Plan

The Company grants performance share units (“PSUs”) to eligible employees as part of its long-term equity compensation program. The number of shares of the Company’s common stock issued to settle PSUs ranges from 0% to 200% of the number of PSUs awarded and is determined based on certain performance criteria over a three-year measurement period. The performance criteria for PSUs are based on a combination of the Company’s annualized Total Shareholder Return (“TSR”) for the performance period and the relative performance of the Company’s TSR compared with the annualized TSR of certain peer companies for the performance period. Compensation expense for PSUs is recognized within general and administrative and exploration expense over the vesting periods of the respective awards.

Total compensation expense recorded for PSUs for the three months ended June 30, 2017, and 2016, was $1.7 million and $3.0 million, respectively, and $4.2 million and $5.9 million for the six months ended June 30, 2017, and 2016, respectively. As of June 30, 2017, there was $10.6 million of total unrecognized compensation expense related to non-vested PSU awards, which is being amortized through 2019. There were no material changes to the outstanding and non-vested PSUs during the six months ended June 30, 2017.

Subsequent to June 30, 2017, the Company granted 977,731 PSUs with a fair value of $15.5 million. These PSUs generally vest on the third anniversary of the date of the grant. Also, subsequent to June 30, 2017, the Company settled PSUs that were granted in 2014 with no shares issued upon settlement as the grant settled at a zero multiplier.

Restricted Stock Units Under the Equity Incentive Compensation Plan

The Company grants restricted stock units (“RSUs”) as part of its long-term equity compensation program. Each RSU represents a right to receive one share of the Company’s common stock upon settlement of the award at the end of the specified vesting period. Compensation expense for RSUs is recognized within general and administrative expense and exploration expense over the vesting periods of the respective awards.

Total compensation expense recorded for RSUs was $2.1 million and $3.3 million for the three months ended June 30, 2017, and 2016, respectively, and $4.6 million and $6.5 million for the six months ended June 30, 2017, and 2016, respectively. As of June 30, 2017, there was $9.8 million of total unrecognized compensation expense related to non-vested RSU awards, which is being amortized through 2019. There were no material changes to the outstanding and non-vested RSUs during the six months ended June 30, 2017.

Subsequent to June 30, 2017, the Company granted 1,010,298 RSUs with a fair value of $16.8 million. These RSUs generally vest one-third of the total grant on each of the next three anniversary dates of the grant. Also, subsequent to June 30, 2017, the Company settled 243,951 RSUs that related to awards granted in previous years. The Company and the majority of grant participants mutually agreed to net share settle a portion of the awards to cover income and payroll tax withholdings, as provided for in the plan document and award agreements. As a result, the Company issued 169,891 net shares of common stock upon settlement of the awards. The remaining 74,060 shares were withheld to satisfy income and payroll tax withholding obligations that occurred upon delivery of the shares underlying those RSUs.

Director Shares

During the second quarter of 2017, the Company issued 71,573 shares of its common stock and 8,794 RSUs to its non-employee directors, under the Company’s Equity Incentive Compensation Plan, which fully vest on December 31, 2017. The Company issued 53,473 shares of its common stock to its non-employee directors during the second quarter of 2016.

Employee Stock Purchase Plan

Under the Company’s Employee Stock Purchase Plan (“ESPP”), eligible employees may purchase shares of the Company’s common stock through payroll deductions of up to 15 percent of eligible compensation, without accruing in excess of $25,000 in value from purchases for each calendar year. The purchase price of the stock is 85 percent of the lower of the fair market value of the stock on either the first or last day of the purchase period. The ESPP is intended to qualify under Section 423 of the Internal Revenue Code (“IRC”). There were 123,678 and 140,853 shares issued under the ESPP during the second quarters of 2017 and 2016, respectively. The fair value of ESPP grants is measured at the date of grant using the Black-Scholes option-pricing model.