Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Amortized Cost	$ 2,739
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	$ 0
Premium Receivable, Allowance for Credit Loss	$ 13	$ 17
Preferred stock, par value	$ 0.01
Preferred stock, shares authorized	20,000,000
Preferred Stock, Shares Issued	0
Common stock, par value	$ 0.01
Common stock, shares authorized	130,000,000
Common stock, shares issued	46,477,067	45,865,081
Common stock, shares outstanding	46,197,102
Treasury stock, shares	279,965	55,942
Equity investments in pooled funds [Member]
Other Investment Not Readily Marketable, Fair Value	$ 600	$ 544
Variable Interest Entity, Primary Beneficiary [Member]
Long-term debt	$ 4,264	$ 4,324