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Subsequent Events (Details) - USD ($)
12 Months Ended
Feb. 15, 2019
Jan. 31, 2019
Jan. 23, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]            
Decrease in other assets related to BlackRock Series C Preferred Stock transferred       $ (108,000,000) $ (647,000,000) $ 744,000,000
Subsequent Event            
Debt Instrument [Line Items]            
Decrease in other assets related to BlackRock Series C Preferred Stock transferred   $ (45,000,000)        
Decrease in other liabilities related to BlackRock Series C Preferred Stock transferred   $ (45,000,000)        
Subsequent Event | Senior Notes | Senior Notes Maturing January 2024            
Debt Instrument [Line Items]            
Debt issued $ 300,000,000   $ 750,000,000      
Debt Instrument, Maturity Date Jan. 23, 2024   Jan. 23, 2024      
Aggregate Outstanding Principal $ 1,050,000,000          
Stated interest rate 3.50%   3.50%      
BlackRock Series C Preferred Stock | Subsequent Event            
Debt Instrument [Line Items]            
LTIP - Number of preferred stock shares transferred (shares)   143,458        
LTIP - Total preferred shares held at end of period (shares)   0