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Stockholders' Equity - Share Repurchase Program (Details) - Common Stock - USD ($)
6 Months Ended
Jun. 30, 2017
Jul. 25, 2017
Apr. 18, 2016
Share Repurchase Program 2016      
Equity, Class of Treasury Stock [Line Items]      
Stock repurchase program, authorized amount     $ 1,000,000,000
Stock repurchased during period (in shares) 1,929,620    
Stock repurchased during period $ 350,000,000    
Stock repurchase program, remaining authorized repurchase amount $ 350,000,000    
Subsequent Event | Share Repurchase Program 2017      
Equity, Class of Treasury Stock [Line Items]      
Stock repurchase program, authorized amount   $ 2,000,000,000