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Derivatives (Narrative) (Details)
 6 Months Ended 60 Months Ended
Jun. 30, 2019
USD ($)
GBP (£)
Item
Dec. 31, 2005
Item
Dec. 31, 2018
USD ($)
Dec. 31, 2001
Item
Derivative [Line Items]        
Number of equity index put option contracts | Item   7   7
Number of equity indices | Item   2    
Equity index put option liability $ 8,374,000   $ 11,958,000  
Equity Index Put Option Contracts [Member]        
Derivative [Line Items]        
Derivative expiration date Jun. 09, 2017      
Equity index put option liability $ 0      
Equity Index Put Option Contracts [Member] | Fair Value [Member]        
Derivative [Line Items]        
Equity index put option liability $ 8,374,000   $ 11,958,000  
Equity Index Put Option Contract, Expired On July 1, 2019 [Member]        
Derivative [Line Items]        
Number of equity index put option contracts | Item 1      
Present value of theoretical maximum payouts $ 80,993,000      
Equity Index Put Option Contract, Expired Prior To June 30, 2019 [Member]        
Derivative [Line Items]        
Number of equity index put option contracts | Item 2      
Standard & Poor's 500 Index [Member]        
Derivative [Line Items]        
Number of equity index put option contracts | Item 4      
Theoretical maximum payout occurs based on index value $ 0      
Discount factor 3.00%      
Present value of theoretical maximum payouts $ 353,283,000      
Settlement amount if contracts had expired $ 0      
Equity Index If Contracts Had Expired 2,941.76      
Standard & Poor's 500 Index [Member] | Maximum [Member]        
Derivative [Line Items]        
Estimates the probability of equity index put option contract will falling below the strike price on the exercise date 1.00%      
FTSE 100 Index [Member]        
Derivative [Line Items]        
Number of equity index put option contracts | Item 1      
Theoretical maximum payout occurs based on index value $ 0      
Discount factor 3.00%      
Present value of theoretical maximum payouts $ 40,113,000      
Settlement amount if contracts had expired $ 0      
Equity Index If Contracts Had Expired | £ 7,425.63      
FTSE 100 Index [Member] | Maximum [Member]        
Derivative [Line Items]        
Estimates the probability of equity index put option contract will falling below the strike price on the exercise date 9.00%