SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 1,915,278	$ 1,776,465
Property and equipment, accumulated depreciation	$ 62,703	$ 54,275
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 1.00
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	67,799,229	67,434,257
Common stock, shares outstanding (in shares)	44,869,015	44,504,043
Treasury stock, shares (in shares)	22,930,214	22,930,214
PARENT COMPANY
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 42,747	$ 58,812
Property and equipment, accumulated depreciation	$ 1,562	$ 1,494
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	67,799,229	67,434,257
Common stock, shares outstanding (in shares)	44,869,015	44,504,043
Treasury stock, shares (in shares)	22,930,214	22,930,214