Interim Unaudited Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Available-for-sale debt securities, amortized cost basis	$ 4,247.6	$ 4,060.9
Available-for-sale equity securities, amortized cost basis	$ 27.0	$ 28.4
Common stock par value (USD per share)	$ 5,000	$ 5,000
Common stock shares authorized (in shares)	1,000	1,000
Common stock shares issued (in shares)	500	500