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Notes Payable - 4.25% Notes (Details)
Oct. 01, 2013
Aug. 02, 2011
USD ($)
day
trading_day
$ / shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]        
Notes repurchase amount     $ 230,000,000 $ 549,000,000
4.25% unsecured convertible notes due August 2018 | Convertible Debt        
Debt Instrument [Line Items]        
Amount of debt instrument   $ 300,000,000.0    
Stated interest rate (as percent)   4.25% 4.25%  
Percentage of principal received in cash   100.00%    
Debt discount rate   92.818%    
Unamortized discount   $ 22,000,000    
Conversion ratio   0.046387    
Conversion price (in dollars per share) | $ / shares   $ 21.56    
If-converted percentage in excess of price 130.00%      
4.25% unsecured convertible notes due August 2018 | Convertible Debt | Debt Conversion, One        
Debt Instrument [Line Items]        
Number of trading days | trading_day   20    
Period of consecutive trading days | day   10    
If-converted percentage in excess of price   130.00%    
4.25% unsecured convertible notes due August 2018 | Convertible Debt | Debt Conversion, Two        
Debt Instrument [Line Items]        
Period of consecutive trading days | day   30    
Threshold consecutive business days   5 days    
Ratio of trading price per $1000 principal amount   98.00%