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Common Stock (Details) - USD ($)
 3 Months Ended 66 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Dec. 31, 2016
Oct. 28, 2015
Oct. 23, 2014
Dec. 05, 2013
Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]                
Common stock, outstanding 59,255,759   59,255,759 59,946,984        
Common stock, issued 117,034 198,127            
Stock repurchased (in US dollars) $ 53,307,000 $ 26,076,000            
Common Stock [Member]                
Equity, Class of Treasury Stock [Line Items]                
Number of shares repurchased     12,200,000          
Stock repurchased (in US dollars) 8,000 $ 6,000 $ 537,700,000          
Remaining authorized repurchase amount (in US dollars) $ 112,300,000   $ 112,300,000          
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 800,000              
Stock repurchased (in US dollars) $ 53,300,000