SCHEDULE II-CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) - Balance Sheets (Parenthietical) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 1,518,156	$ 1,448,204
Property and equipment, accumulated depreciation	$ 38,447	$ 34,365
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)	66,474,342	66,032,929
Common stock, shares outstanding (in shares)	43,544,128	43,102,715
Treasury stock, shares (in shares)	22,930,214	22,930,214
PARENT COMPANY
CONDENSED BALANCE SHEETS (Parenthetical)
Available-for-sale, amortized cost	$ 45,178	$ 46,226
Property and equipment, accumulated depreciation	$ 765	$ 528
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)	66,474,342	66,032,929
Common stock, shares outstanding (in shares)	43,544,128	43,102,715
Treasury stock, shares (in shares)	22,930,214	22,930,214