CONSOLIDATED BALANCE SHEET (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2023		Dec. 31, 2022
Statement of Financial Position [Abstract]
Amortized cost of investments	$ 22,667.6	[1]	$ 22,043.0	[2]
Common stock, par value (in dollars per share)	$ 0.001		$ 0.001
Common stock, shares authorized (in shares)	1,000		700
Common stock, shares issued (in shares)	582		519
Common stock, shares outstanding (in shares)	582		519

[1] As of December 31, 2023, the estimated net unrealized loss for federal tax purposes was $0.4 billion based on a tax cost basis of $22.5 billion. As of December 31, 2023, the estimated aggregate gross unrealized loss for federal income tax purposes was $1.7 billion and the estimated aggregate gross unrealized gain for federal income tax purposes was $1.3 billion.     

[2] As of December 31, 2022, the estimated net unrealized loss for federal tax purposes was $0.2 billion based on a tax cost basis of $22.0 billion. As of December 31, 2022, the estimated aggregate gross unrealized loss for federal income tax purposes was $1.2 billion and the estimated aggregate gross unrealized gain for federal income tax purposes was $1.0 billion.