Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Available-for-sale debt securities, amortized cost basis	$ 4,060.9		$ 3,390.6
Available-for-sale equity securities, amortized cost basis	$ 28.4		$ 12.1
Common stock par value (USD per share)	$ 5,000	$ 5,000	$ 5,000
Common stock shares authorized (in shares)	1,000us-gaap_CommonStockSharesAuthorized	1,000us-gaap_CommonStockSharesAuthorized	1,000us-gaap_CommonStockSharesAuthorized
Common stock shares issued (in shares)	500us-gaap_CommonStockSharesIssued	500us-gaap_CommonStockSharesIssued	500us-gaap_CommonStockSharesIssued

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Face amount or stated value per share of common stock.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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