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Common Stock (Details) - USD ($)
 3 Months Ended 12 Months Ended 99 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2019
Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]            
Common stock, outstanding (in shares) 56,219,831 56,219,831 55,983,763   56,219,831  
Dividend paid (in dollars per share) $ 0.08          
Stock repurchased (in US dollars)     $ 216,244,000 $ 90,821,000    
Common Stock            
Equity, Class of Treasury Stock [Line Items]            
Number of shares repurchased (in shares)   0     15,900,000  
Stock repurchased (in US dollars)       $ 10,000 $ 791,500,000  
Remaining authorized repurchase amount (in US dollars) $ 158,500,000 $ 158,500,000     158,500,000  
Common Stock | RepurchaseProgramSept262011            
Equity, Class of Treasury Stock [Line Items]            
Stock repurchase, authorized amount (in US dollars)           $ 100,000,000.0
Common Stock | RepurchaseProgramSep2011toDec2019            
Equity, Class of Treasury Stock [Line Items]            
Stock repurchase, authorized amount (in US dollars) $ 950,000,000.0 $ 950,000,000.0     $ 950,000,000.0