Bank Line of Credit (Details) - Revolving line of credit - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Oct. 28, 2015	Sep. 30, 2015
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
Outstanding amount of letters of credit under the bank revolving line of credit		2
Available amount of letters of credit under the bank revolving line of credit		23
Borrowings outstanding under the bank revolving line of credit		0
Fair value of bonds pledged as collateral on the bank revolving line of credit		111
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		$ 299
Outstanding amount of letters of credit under the bank revolving line of credit		1
Available amount of letters of credit under the bank revolving line of credit		24
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
Subsequent event | Indemnity
Bank Line of Credit
Expiration date of the bank revolving line of credit	Nov. 03, 2020
Subsequent event description	On October 28, 2015, Indemnity extended the maturity date of the revolving credit facility to November 3, 2020.
Subsequent event | Exchange
Bank Line of Credit
Maximum borrowing capacity under the bank revolving line of credit	$ 400
Expiration date of the bank revolving line of credit	Oct. 28, 2020
Subsequent event description	On October 28, 2015, the Exchange amended the bank revolving line of credit to $400 million and extended the maturity date to October 28, 2020. The increase in the line of credit to $400 million was driven by both favorable credit market conditions and increased need based on Exchange's growth in business.