Share Based Compensation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Valuation And Expense Information [Abstract]
Total cost of share based payment plans	$ 1,706	$ 3,963	$ 5,756	$ 8,247
Amounts capitalized in oil and gas properties and equipment	442	1,283	1,789	2,738
Amounts charged against income before income tax benefit	1,264	2,680	3,967	5,509
Amount of related income tax benefit recognized in income before valuation allowance	$ 503	$ 1,120	$ 1,579	$ 2,303
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit			$ 49.05		$ 49.05	$ 25.68
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit			$ 98.87		$ 98.87	$ 98.87
Expired or forfeited			(109)		(171)
Exercise Price, Lower Range Limit Expired or Forfeited			$ 50.48		$ 25.68
Exercise Price, Upper Range Limit Expired or Forfeited			$ 75.18		$ 75.18
Number Of Options [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options			519	690	690
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options	410		410		519	690