Short-term Borrowings (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Borrowings
Amount of the company's liability including accrued interest	$ 500	$ 501
Repurchase Agreements
Borrowings
Fair value of securities pledged	208	518
Amount of the company's liability including accrued interest	200	501
Weighted average annual interest rate on the FHLB advances (as a percent)	0.40%	0.40%
Federal Home Loan Bank (FHLB) Advances
Borrowings
Fair value of securities pledged	389
Amount of the company's liability including accrued interest	$ 300
Weighted average annual interest rate on the FHLB advances (as a percent)	0.30%