Bank Line of Credit (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Indemnity
Bank Line of Credit
Maximum borrowing capacity under the line of credit	$ 100
Fair value of bonds pledged as collateral on the line of credit	112
Exchange
Bank Line of Credit
Maximum borrowing capacity under the line of credit	300
Fair value of bonds pledged as collateral on the line of credit	$ 333