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Long-term Debt - Cash Convertible Senior Notes Due 2019 (Details)
$ / shares in Units, shares in Millions
1 Months Ended 3 Months Ended
Jul. 31, 2017
USD ($)
$ / shares
Jul. 31, 2014
USD ($)
Jun. 30, 2014
USD ($)
$ / shares
$ / share
shares
Dec. 31, 2018
Convertible Senior Notes        
Debt Instrument [Line Items]        
Stated interest rate     2.125%  
Effective interest rate       8.00%
2019 Convertible Notes        
Debt Instrument [Line Items]        
Stated interest rate     2.125%  
Convertible senior notes issued and settled | $   $ 30,000,000 $ 200,000,000  
Convertible senior notes repurchased and retired | $ $ 35,000,000.0      
2024 Convertible Notes        
Debt Instrument [Line Items]        
Stated interest rate 2.875%      
Convertible senior notes issued and settled | $ $ 34,400,000      
Common Stock Class A Non-voting | 2019 Convertible Notes        
Debt Instrument [Line Items]        
Conversion rate per $1000 principal amount     62.2471  
Conversion price, per share (in dollars per share) | $ / shares     $ 16.065  
Common Stock Class A Non-voting | 2024 Convertible Notes        
Debt Instrument [Line Items]        
Conversion rate per $1000 principal amount 100      
Conversion price, per share (in dollars per share) | $ / shares $ 10.00      
2019 Convertible Notes Hedges — Level 2 | Common Stock Class A Non-voting        
Debt Instrument [Line Items]        
Notional amount (shares) | shares     12.1  
Strike price (dollars per share) | $ / share     16.065  
Net-share-settled Warrants | Common Stock Class A Non-voting        
Debt Instrument [Line Items]        
Number of securities called (shares) | shares     12.1  
Strike price for warrants (in dollars per share) | $ / shares     $ 20.83