Bank Line of Credit (Details) (Revolving line of credit, USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Indemnity
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	$ 100
Fair value of bonds pledged as collateral on the bank revolving line of credit	108
Exchange
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	300
Fair value of bonds pledged as collateral on the bank revolving line of credit	$ 323