Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost	$ 3,038,000,000	$ 3,187,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Premium Receivable, Allowance for Credit Loss	$ 14,352,000
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued	45,865,081	45,571,743
Common stock, shares outstanding	45,779,023	45,555,400
Treasury stock, shares	86,058	16,343
Variable Interest Entity, Primary Beneficiary [Member]
Long-term debt	$ 4,092,000,000	$ 4,351,000,000
Equity investments in pooled funds [Member]
Other Investment Not Readily Marketable, Fair Value	$ 317,000,000	$ 432,000,000