Long-Term Debt and Credit Facilities - Credit Facilities (Details) (USD $)	Sep. 30, 2014 2009 Strip Coverage Facility AGM	Jun. 30, 2014 2009 Strip Coverage Facility AGM	Jul. 01, 2009 2009 Strip Coverage Facility AGM	Oct. 25, 2013 AGUS Revolving Credit Facility
Credit facilities
Liquidity claims on gross exposure	$ 1,200,000,000
Cumulative strip par exposure terminated	1,400,000,000
Maximum credit facility		495,000,000	1,000,000,000	225,000,000
Fixed interest rate as percentage of federal rate				100.00%
Amount to be added to net worth requirement if borrowing capacity is reduced	0
Maximum debt-to-capital ratio which is required to be maintained (as a percent)	30.00%
Minimum net worth which is required to be maintained (as a percent)	75.00%
Minimum percentage of aggregate consolidate net income (or loss) for July 1, 2009 through June 30, 2014 to be added to net worth requirement	25.00%
Maximum commitment amount used for financial covenant calculation	$ 1,000,000,000