Derivative Financial Instruments - Hedged Items in Fair Value (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value, Available for Sale Debt Securities	$ 3,831.6
Carrying value, Senior Notes and Long-Term Subordinated Debt	1,248.8
Carrying value, Total	5,080.4
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	99.4
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	29.3
Cumulative Hedge Accounting Basis Adjustment, Total	$ 128.7