SCHEDULE II - BALANCE SHEET Parenthetical (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available for sale, amortized cost	$ 19,803.1	$ 18,947.0
Equity securities cost	$ 580.7	$ 447.4
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	8,000,000,000	8,000,000,000
Common stock, shares issued (in shares)	173,753,614	184,028,511
Common stock, shares outstanding (in shares)	173,753,614	184,028,511
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available for sale, amortized cost	$ 0.0	$ 5.0
Equity securities cost	$ 166.5	$ 247.3
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	8,000,000,000	8,000,000,000
Common stock, shares issued (in shares)	173,753,614	184,028,511
Common stock, shares outstanding (in shares)	173,753,614	184,028,511