SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets (Parenthietical) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 1,596,227	$ 1,518,156
Property and equipment, accumulated depreciation	$ 41,999	$ 38,447
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	66,874,911	66,474,342
Common stock, shares outstanding (in shares)	43,944,697	43,544,128
Treasury stock, shares (in shares)	22,930,214	22,930,214
PARENT COMPANY
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 45,901	$ 45,178
Property and equipment, accumulated depreciation	$ 1,523	$ 765
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	66,874,911	66,474,342
Common stock, shares outstanding (in shares)	43,944,697	43,544,128
Treasury stock, shares (in shares)	22,930,214	22,930,214