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Senior Notes, Convertible Notes and Trust Preferred Securities - Schedule of Long-term Debt Instruments (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
May 31, 2014
Jun. 30, 2013
Debt Instrument [Line Items]        
Unamortized debt issuance costs $ (7,258,000)      
Senior Notes [Member]        
Debt Instrument [Line Items]        
Principal amount of debt component 500,000,000 $ 500,000,000    
Unamortized debt discounts (1,507,000) (1,780,000)    
Unamortized debt issuance costs (3,295,000) (3,858,000)    
Long-term debt 495,198,000 494,362,000    
Senior Notes [Member] | Senior Notes Due 2024 [Member]        
Debt Instrument [Line Items]        
Principal amount of debt component $ 250,000,000 $ 250,000,000    
Debt interest rate percentage 4.40% 4.40% 4.40%  
Issue Price 99.883%      
Senior Notes [Member] | Senior Notes Due 2023 [Member]        
Debt Instrument [Line Items]        
Principal amount of debt component $ 250,000,000 $ 250,000,000    
Debt interest rate percentage 4.25% 4.25%   4.25%
Issue Price 99.026%