Mortgage Servicing Rights (Schedule of Sensitivity Analysis of Fair Value) (Details)
 6 Months Ended 12 Months Ended
Jun. 30, 2016
USD ($)
$ / loan
Dec. 31, 2015
USD ($)
$ / loan
Dec. 31, 2014
USD ($)
Servicing Assets at Fair Value [Line Items]      
Option adjusted spread, Actual (percent)   8.24% 8.88%
Option adjusted spread, Fair value due to 10% adverse change   $ 287,000,000 $ 250,000,000
Option adjusted spread, Fair value due to 20% adverse change   $ 279,000,000 $ 243,000,000
Constant prepayment rate, Actual (percent)   12.63% 14.98%
Constant prepayment rate, Fair value due to 10% adverse change   $ 285,000,000 $ 253,000,000
Constant prepayment rate, Fair value due to 20% adverse change   275,000,000 245,000,000
Weighted average cost to service per loan, Actual   71.86 74.49
Weighted average cost to service per loan, Fair value due to 10% adverse change   292,000,000 258,000,000
Weighted average cost to service per loan, Fair value due to 20% adverse change   $ 288,000,000 $ 255,000,000
Mortgage servicing rights      
Servicing Assets at Fair Value [Line Items]      
Option adjusted spread, Actual (percent) 8.37% 8.24%  
Option adjusted spread, Fair value due to 10% adverse change $ 293,000,000 $ 287,000,000  
Option adjusted spread, Fair value due to 20% adverse change $ 285,000,000 $ 279,000,000  
Constant prepayment rate, Actual (percent) 16.38% 12.63%  
Constant prepayment rate, Fair value due to 10% adverse change $ 287,000,000 $ 285,000,000  
Constant prepayment rate, Fair value due to 20% adverse change 274,000,000 275,000,000  
Weighted average cost to service per loan, Fair value due to 10% adverse change 297,000,000 292,000,000  
Weighted average cost to service per loan, Fair value due to 20% adverse change $ 293,000,000 $ 288,000,000  
Sensitivity Analysis of Fair Value, Transferor's Interests in Transferred Financial Assets, Impact of Other than 10 or 20 Percent Adverse Change in Other Assumption, Per Loan | $ / loan 70.99 71.86