Bank Line of Credit (Details) (Revolving line of credit, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014
Indemnity
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	$ 100
Maximum letter of credit sublimit under the bank revolving line of credit	25
Expiration date of the bank revolving line of credit	Nov. 03, 2018
Available borrowing capacity under the bank revolving line of credit, due to outstanding letters of credit	98.2
Outstanding amount of letters of credit under the bank revolving line of credit	1.8
Available amount of letters of credit under the bank revolving line of credit	23.2
Borrowings outstanding under the bank revolving line of credit	0
Fair value of bonds pledged as collateral on the bank revolving line of credit	113
Exchange
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	300
Maximum letter of credit sublimit under the bank revolving line of credit	25
Expiration date of the bank revolving line of credit	Oct. 25, 2018
Available borrowing capacity under the bank revolving line of credit, due to outstanding letters of credit	298.9
Outstanding amount of letters of credit under the bank revolving line of credit	1.1
Available amount of letters of credit under the bank revolving line of credit	23.9
Borrowings outstanding under the bank revolving line of credit	0
Fair value of bonds pledged as collateral on the bank revolving line of credit	$ 335