Derivative Instruments and Hedging Activities (Basis Adjustments for Hedged Items) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Derivative Instruments, Gain (Loss) [Line Items]
Total debt securities	$ 11,452.0	$ 11,759.0
Cumulative amount of fair value hedging adjustment included in the carrying amount of the hedged assets	50.5
Amount of hedging	3,500.0
Fair Value Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Par value of hedged assets	12,039.0	12,389.0
Carrying amount of hedged assets	11,761.0	12,209.0
Total debt securities	11,000.0
Cumulative amount of fair value hedging adjustment included in the carrying amount of the hedged assets	$ (278.0)	$ (180.0)