Investments Investment Portfolio by Major Security Type (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Investments, Debt and Equity Securities [Abstract]
Net unsettled security transactions offset in other liabilities	$ 159.7	$ 31.3	$ 235.8
Securities in the portfolio of a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions	$ 700.0	$ 1,900.0	$ 1,100.0