Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Difference between carrying amounts measured at fair value and aggregate unpaid principal amount
Carrying amount, Total	$ 1,155	$ 2,174
Mortgage loans held for sale
Difference between carrying amounts measured at fair value and aggregate unpaid principal amount
Carrying amount, Total	1,155	2,174
Carrying amount, Loans 90 days or more past due and on non-accrual status	15	17
Aggregate unpaid principal balance, Total	1,143	2,126
Aggregate unpaid principal balance, Loans 90 days or more past due and on non-accrual status	22	25
Difference, Total	12	48
Difference, Loans 90 days or more past due and on non-accrual status	$ (7)	$ (8)