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Long-Term Debt - Principal Payment Schedule (Details) - USD ($)
Sep. 30, 2018
May 14, 2018
Jul. 31, 2017
Jun. 30, 2014
Debt Instrument [Line Items]        
Total $ 512,554,000      
Less Than 1 Year 195,196,000      
1 - 3 Years 428,000      
3 - 5 Years 428,000      
More Than 5 Years 316,502,000      
2019 Convertible Notes        
Debt Instrument [Line Items]        
Stated interest rate       2.125%
2024 Convertible Notes        
Debt Instrument [Line Items]        
Total     $ 143,750,000.00  
Stated interest rate     2.875%  
Senior Notes | 2019 Convertible Notes        
Debt Instrument [Line Items]        
Total 195,000,000      
Less Than 1 Year 195,000,000      
1 - 3 Years 0      
3 - 5 Years 0      
More Than 5 Years 0      
Senior Notes | 2024 Convertible Notes        
Debt Instrument [Line Items]        
Total 143,750,000      
Less Than 1 Year 0      
1 - 3 Years 0      
3 - 5 Years 0      
More Than 5 Years 143,750,000      
Senior Notes | 2025 Convertible Notes        
Debt Instrument [Line Items]        
Total 172,500,000 $ 172,500,000.0    
Less Than 1 Year 0      
1 - 3 Years 0      
3 - 5 Years 0      
More Than 5 Years 172,500,000      
Stated interest rate   2.375%    
Unsecured Notes | 8.5% unsecured notes due 2024        
Debt Instrument [Line Items]        
Total 1,304,000      
Less Than 1 Year 196,000      
1 - 3 Years 428,000      
3 - 5 Years 428,000      
More Than 5 Years $ 252,000      
Stated interest rate 8.50%