Legal Proceedings, Contingencies and Commitments (Commitments) (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Commitments [Line Items]      
Letter of credit, reserve amount (less than for $0.1 million) $ 3 $ 2 $ 1
Mortgage loans interest-rate lock commitments      
Commitments [Line Items]      
Commitments to extend credit 6,398 3,792 2,172
HELOC      
Commitments [Line Items]      
Commitments to extend credit 161 150 88
Other consumer commitments      
Commitments [Line Items]      
Commitments to extend credit 33 22 7
Warehouse loan commitments      
Commitments [Line Items]      
Commitments to extend credit 779 871 827
Standby and commercial letters of credit      
Commitments [Line Items]      
Commitments to extend credit 18 13 10
Commercial and industrial      
Commitments [Line Items]      
Commitments to extend credit 157 151 276
Other commercial commitments      
Commitments [Line Items]      
Commitments to extend credit $ 723 $ 497 $ 169