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Debt - Secured Unsecured Notes (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Aug. 31, 2014
Debt Instrument [Line Items]      
Amortization of deferred financing costs $ 1,300,000 $ 2,400,000  
Convertible Debt | Exchangeable Senior Notes due August 15, 2019      
Debt Instrument [Line Items]      
Debt instrument face amount     $ 250,000,000.0
Stated rate 2.625%   2.625%
Interest expense, debt $ 2,400,000 2,400,000  
Contractual interest expense 1,600,000 1,600,000  
Accretion of debt discount 700,000 700,000  
Amortization of deferred financing costs $ 100,000 100,000  
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]      
Debt instrument face amount   $ 100,000,000  
Stated rate 3.93% 3.93%  
Senior Notes | Series B Senior Notes due 2027      
Debt Instrument [Line Items]      
Debt instrument face amount   $ 125,000,000  
Stated rate 4.09% 4.09%  
Senior Notes | Series C Senior Notes due 2030      
Debt Instrument [Line Items]      
Debt instrument face amount   $ 125,000,000  
Stated rate 4.18% 4.18%