Stock-Based Compensation Plans (Tables)

6 Months Ended

Jun. 30, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Allocation of Stock-Based Compensation by Functional Category

The following table details the allocation of stock-based compensation to functional expense categories (in thousands):

		Three Months Ended June 30,				Six Months Ended June 30,
		2016		2015		2016		2015
Direct operating expense	$	696	$	654	$	1,284	$	1,540
Brokered natural gas and marketing expense		378		619		894		1,125
Exploration expense		371		751		1,061		1,483
General and administrative expense		15,443		15,953		26,556		27,033
Termination costs		¾		434		¾		1,721
Total stock-based compensation	$	16,888	$	18,411	$	29,795	$	32,902

Summary of Performance Share Unit Awards Activities

The following is a summary of our non-vested PSU awards outstanding at June 30, 2016:

	Number of Units			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2015		262,124		$	64.77
Units granted (a)		413,959			36.64
Units vested		(113,041	)		52.88
Units forfeited		(42,603	)		46.09
Outstanding at June 30, 2016		520,439		$	46.51

^(a) Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero percent and 150% of the performance units granted depending on the total shareholder return ranking compared to the peer companies at the end of the three-year performance period.

Schedule of Share Based Payment Award Performance Stock Awards Valuation Assumptions

The following assumptions were used to estimate the fair value of PSUs granted during first six months 2016 and 2015:

		Six Months Ended June 30,
		2016			2015
Risk-free interest rate		0.94	%		1.0	%
Expected annual volatility		49	%		33	%
Weighted average grant date fair value per unit	$	36.64		$	56.78

Restricted Stock and Restricted Stock Units Outstanding

The following is a summary of the status of our non-vested restricted stock outstanding at June 30, 2016:

	Equity Awards					Liability Awards
	Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2015		436,764		$	59.74		308,737		$	65.80
Granted		940,491			28.18		508,206			36.01
Vested		(255,270	)		45.79		(214,406	)		52.91
Forfeited		(106,120	)		43.45		(49,519	)		40.33
Outstanding at June 30, 2016		1,015,865		$	35.73		553,018		$	45.70

Stock Option and SARs Activities

Information with respect to SARs activity in the six months ended June 30, 2016 is summarized below:

	Shares			Weighted Average Exercise Price
Outstanding at December 31, 2015		1,510,977		$	63.73
Exercised		—			—
Expired/forfeited		(507,377	)		53.16
Outstanding at June 30, 2016		1,003,600		$	69.08