Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost	$ 2,914,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Premium Receivable, Allowance for Credit Loss	$ 18,000,000
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	45,865,081	45,571,743
Common stock, shares outstanding	45,809,139
Treasury stock, shares	55,942	16,343
Equity investments in pooled funds [Member]
Other Investment Not Readily Marketable, Fair Value	$ 453,000,000	$ 432,000,000
Variable Interest Entity, Primary Beneficiary [Member]
Long-term debt	$ 4,002,000,000	$ 4,351,000,000