Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale debt securities, amortized cost basis	$ 3,043.0	$ 2,460.9
Common stock par value	$ 5,000
common stock shares authorized	1,000
common stock shares issued	500
As restated and amended [Member]
Available-for-sale debt securities, amortized cost basis		$ 2,460.9
Common stock par value		$ 5,000
common stock shares authorized		1,000
common stock shares issued		500