Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Jun. 30, 2015
Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Accumulated other comprehensive income (loss), beginning balance $ 2 $ 8 $ (5) $ (2)    
Net unrealized gain (loss), net of tax   (10) 13 (3)    
Reclassifications out of AOCI   4 0 0    
Transfer of net unrealized loss from AFS to HTM   0        
Accumulated other comprehensive income (loss), ending balance (19) 2 8 (5)    
Stockholders' Equity Attributable to Parent 1,599 1,529 1,373 1,426 $ 1,451 $ 1,160
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Accumulated other comprehensive income (loss), beginning balance (3) 0 0 0    
Net unrealized gain (loss), net of tax (44) (5) 0 0    
Reclassifications out of AOCI 7 2 0 0    
Transfer of net unrealized loss from AFS to HTM   0        
Accumulated other comprehensive income (loss), ending balance (40) (3) 0 0    
Stockholders' Equity Attributable to Parent (40) (3) 0   0  
Accumulated Other Comprehensive Income (Loss)            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Net unrealized gain (loss), net of tax (28)          
Reclassifications out of AOCI 7          
Stockholders' Equity Attributable to Parent (19) 2 8 (5) 8 $ (2)
Held-to-Maturity Securities | Accumulated Net Investment Gain (Loss) Attributable to Parent            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Accumulated other comprehensive income (loss), beginning balance 5 0 0 0    
Net unrealized gain (loss), net of tax 0 0 0 0    
Reclassifications out of AOCI (1) 0 0 0    
Transfer of net unrealized loss from AFS to HTM   5        
Accumulated other comprehensive income (loss), ending balance   5 0 0    
Stockholders' Equity Attributable to Parent 4 5 0   0  
Available-for-sale Securities | Accumulated Net Investment Gain (Loss) Attributable to Parent            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Accumulated other comprehensive income (loss), beginning balance 0 8 (5) (2)    
Net unrealized gain (loss), net of tax 16 (5) 13 (3)    
Reclassifications out of AOCI 1 2 0 0    
Transfer of net unrealized loss from AFS to HTM   (5)        
Accumulated other comprehensive income (loss), ending balance   0 8 $ (5)    
Stockholders' Equity Attributable to Parent $ 17 $ 0 $ 8   $ 8