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Common Stock (Details) - USD ($)
 3 Months Ended 78 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Dec. 31, 2017
Oct. 25, 2017
Oct. 28, 2015
Oct. 23, 2014
Dec. 05, 2013
Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]                  
Common stock, outstanding 58,450,685   58,450,685 58,798,428          
Common stock, issued 98,153 117,034              
Stock repurchased (in US dollars) $ 34,485,000 $ 53,307,000              
Common Stock [Member]                  
Equity, Class of Treasury Stock [Line Items]                  
Number of shares repurchased     13,300,000            
Stock repurchased (in US dollars) 0 $ 8,000 $ 609,700,000            
Remaining authorized repurchase amount (in US dollars) $ 140,300,000   $ 140,300,000            
Common Stock [Member] | RepurchaseProgramSep262011                  
Equity, Class of Treasury Stock [Line Items]                  
Stock repurchase, authorized amount (in US dollars)                 $ 100,000,000
Common Stock [Member] | RepurchaseProgramDec052013                  
Equity, Class of Treasury Stock [Line Items]                  
Stock repurchase, authorized amount (in US dollars)               $ 100,000,000  
Common Stock [Member] | RepurchaseProgramOct232014                  
Equity, Class of Treasury Stock [Line Items]                  
Stock repurchase, authorized amount (in US dollars)             $ 250,000,000    
Common Stock [Member] | RepurchaseProgramOct282015                  
Equity, Class of Treasury Stock [Line Items]                  
Stock repurchase, authorized amount (in US dollars)           $ 200,000,000      
Number of shares repurchased 445,896                
Stock repurchased (in US dollars) $ 34,500,000                
Common Stock [Member] | RepurchaseProgramOct252017                  
Equity, Class of Treasury Stock [Line Items]                  
Stock repurchase, authorized amount (in US dollars)         $ 100,000,000