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Nature of Operations and Basis of Presentation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Dec. 15, 2015
Dec. 15, 2014
Nature of Operations        
Restricted Cash and Cash Equivalents, Current $ 2,270 $ 2,270    
Allowance for doubtful accounts 1,700 1,400    
Accumulated depreciation 1,532,200 $ 1,512,100    
Stock Repurchase Program, Remaining Authorized Repurchase Amount $ 72,300      
Percentage Of Expected Natural Gas Requirement Covered Under Contract 43.00%      
Common Stock        
Nature of Operations        
Common stock repurchase authorized     $ 100,000 $ 100,000
Number of shares repurchased (in shares) 709,366 1,881,921    
Treasury Stock Acquired, Average Cost Per Share $ 38.99 $ 53.13    
Other Assets [Member]        
Nature of Operations        
Restricted Cash and Cash Equivalents, Noncurrent   $ 2,300