Bank Line of Credit (Details) (Revolving line of credit, USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Indemnity
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	$ 100
Borrowings outstanding under the bank revolving line of credit	0
Fair value of bonds pledged as collateral on the bank revolving line of credit	110
Exchange
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	300
Borrowings outstanding under the bank revolving line of credit	0
Fair value of bonds pledged as collateral on the bank revolving line of credit	$ 326