Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Fixed maturity securities, held-to-maturity, fair value	$ 507,625	$ 594,661
Fixed maturity securities, available-for-sale, amortized cost	3,363,994	3,130,683
Equity securities, available-for-sale, cost	142,434	132,441
Premiums receivable, allowance for uncollectible accounts	4,478	3,906
Property and equipment, accumulated depreciation and amortization	$ 174,226	$ 169,428
Preferred stock, par value per share	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	98,910,399	98,194,224
Treasury stock, shares	43,181,889	43,030,776