Schedule II - Condensed Financial Information of Registrant (Parent Company) - Balance Sheets (Parenthetical) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Condensed Balance Sheet Statements Captions [Line Items]
Available-for-sale, amortized cost	$ 2,346,267	$ 2,061,467
Available-for-sale, allowance for credit losses	441	397
Property and equipment, accumulated depreciation	$ 75,236	$ 68,682
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	68,219,551	68,072,794
Common stock, shares outstanding (in shares)	45,289,337	45,142,580
Treasury stock, shares (in shares)	22,930,214	22,930,214
Parent Company
Condensed Balance Sheet Statements Captions [Line Items]
Available-for-sale, amortized cost	$ 84,823	$ 60,657
Available-for-sale, allowance for credit losses	0	0
Property and equipment, accumulated depreciation	$ 1,697	$ 1,629
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	68,219,551	68,072,794
Common stock, shares outstanding (in shares)	45,289,337	45,142,580
Treasury stock, shares (in shares)	22,930,214	22,930,214