SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 1,776,465	$ 1,646,411
Property and equipment, accumulated depreciation	$ 54,275	$ 47,676
Common stock, par value (in dollars per share)	$ 0.01	$ 1.00
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	67,434,257	67,078,569
Common stock, shares outstanding (in shares)	44,504,043	44,148,355
Treasury stock, shares (in shares)	22,930,214	22,930,214
PARENT COMPANY
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 58,812	$ 23,184
Property and equipment, accumulated depreciation	$ 1,494	$ 1,426
Common stock, par value (in dollars per share)	$ 0.01	$ 1.00
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	67,434,257	67,078,569
Common stock, shares outstanding (in shares)	44,504,043	44,148,355
Treasury stock, shares (in shares)	22,930,214	22,930,214