Summary of Senior Notes Outstanding (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Senior Notes	$ 1,497.4	$ 1,497.0
Fixed Rate Corporate Senior Notes 3.45 Percent Due November 2020 [Member]
Debt Instrument [Line Items]
Interest Rate	3.45%
Senior Notes	$ 499.7	499.6
Fixed Rate Corporate Senior Notes 3.38 Percent Due August 2021 [Member]
Debt Instrument [Line Items]
Interest Rate	3.38%
Senior Notes	$ 498.7	498.5
Fixed Rate Corporate Senior Notes Two Point Three Eight Percent Due August Twenty Twenty Two [Member]
Debt Instrument [Line Items]
Interest Rate	2.38%
Senior Notes	$ 499.0	$ 498.9