Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Available-for-sale debt securities, amortized cost basis	$ 3,402.6				$ 2,798.1
Common stock par value (USD per share)	$ 5,000	$ 5,000	$ 5,000	$ 5,000	$ 5,000
Common stock shares authorized (in shares)	1,000	1,000	1,000	1,000	1,000
Common stock shares issued (in shares)	500	500	500	500	500