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Debt - Senior Unsecured Notes (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Mar. 31, 2015
Aug. 31, 2014
Debt Instrument [Line Items]            
Amortization of deferred financing costs $ 1,500,000 $ 1,400,000 $ 3,900,000 $ 2,500,000    
Convertible Debt | Exchangeable Senior Notes due August 15, 2019            
Debt Instrument [Line Items]            
Stated rate 2.625%   2.625%     2.625%
Interest expense debt $ 2,500,000   $ 4,900,000      
Contractual interest rate 1,700,000   3,300,000      
Accretion of debt discount 600,000   1,300,000      
Amortization of deferred financing costs $ 200,000   $ 300,000      
Debt instrument face amount           $ 250,000,000.0
Senior Notes | Series A, Series B, and Series C Senior Unsecured Notes [Member]            
Debt Instrument [Line Items]            
Debt instrument face amount         $ 350,000,000  
Senior Notes | Series A Senior Notes due 2025            
Debt Instrument [Line Items]            
Stated rate 3.93%   3.93%   3.93%  
Debt instrument face amount         $ 100,000,000  
Senior Notes | Series B Senior Notes due 2027            
Debt Instrument [Line Items]            
Stated rate 4.09%   4.09%   4.09%  
Debt instrument face amount         $ 125,000,000  
Senior Notes | Series C Senior Notes due 2030            
Debt Instrument [Line Items]            
Stated rate 4.18%   4.18%   4.18%  
Debt instrument face amount         $ 125,000,000