Fair Value Measurements (Assets Measured at Fair Value on a Non-recurring Basis) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Loans held-for-sale $ 3,071 $ 2,541       $ 1,196               $ 3,071   $ 2,541 $ 1,196  
Total assets at fair value 3,159 2,652       1,407       $ 1,379       3,159   2,652 1,407 $ 1,379
Net gain (loss) on sales of repossessed assets 90 46 $ 68 $ 83 $ 91 53 $ 52 $ 55 $ 45 $ 45 $ 75 $ 144 $ 137 165 $ 174 288 206 402
Nonrecurring                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Repossessed assets 19 17       19               19   17 19  
Total assets at fair value 61 67       120               61   67 120  
Fair value gains (losses) on repossessed assets (3)     0                   (2) (1) (2) (4) (10)
Net gain (loss) on sales of repossessed assets 0     1                   1 1 (2) (5) (26)
Nonrecurring | Interest Income                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Fair value gains (losses) on loans (1)     (1)                   (1) (1) (2) (4) (4)
Nonrecurring | Provision for loan losses                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Fair value gains (losses) on loans (9)     $ (21)                   (20) $ (56) (84) (49) $ (155)
Nonrecurring | Level 2                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Repossessed assets 0 0       0               0   0 0  
Total assets at fair value 16 8       27               16   8 27  
Nonrecurring | Level 3                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Repossessed assets 19 17       19               19   17 19  
Total assets at fair value 45 59       93               45   59 93  
Nonrecurring | Residential first mortgage loans                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Loans held-for-sale 16 8       27               16   8 27  
Impaired loans held-for-investment 25 40       74               25   40 74  
Nonrecurring | Residential first mortgage loans | Level 2                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Loans held-for-sale 16 8       27               16   8 27  
Impaired loans held-for-investment 0 0       0               0   0 0  
Nonrecurring | Residential first mortgage loans | Level 3                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Loans held-for-sale 0 0       0               0   0 0  
Impaired loans held-for-investment 25 40       $ 74               25   40 $ 74  
Nonrecurring | Commercial and industrial                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Impaired loans held-for-investment 1 2                       1   2    
Nonrecurring | Commercial and industrial | Level 2                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Impaired loans held-for-investment 0 0                       0   0    
Nonrecurring | Commercial and industrial | Level 3                                    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                    
Impaired loans held-for-investment $ 1 $ 2                       $ 1   $ 2