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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Long-term Debt    
Transfers between Levels 1 and 2, description The Company did not have material transfers between Levels 1 and 2 during the nine months ended September 30, 2017 or 2016.  
Restricted investments, amortized cost $ 48,775  
Long-term debt, carrying amount 886,276 $ 759,519
2017 Senior Notes [Member]    
Long-term Debt    
Long-term debt, fair value 537,500  
Long-term debt, carrying amount 492,545  
2010 Senior Notes [Member]    
Long-term Debt    
Long-term debt, fair value   302,600
Long-term debt, carrying amount   298,120
Convertible Notes [Member]    
Long-term Debt    
Long-term debt, fair value 231,900 228,400
Long-term debt, carrying amount 159,314 $ 152,668
Significant Other Observable Inputs (Level 2) [Member]    
Long-term Debt    
Restricted investments, fair value $ 48,700