Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized cost	$ 526,762	$ 404,977
Held-to-maturity securities, fair value	$ 1,589,183	$ 1,438,960
Preferred stock, par value	$ 1.00	$ 1.00
Preferred stock, shares authorized	100,000	100,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Class A Common Stock [Member]
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares authorized	10,000,000	10,000,000
Common stock, shares issued	3,600,729	3,600,729
Class B Common Stock [Member]
Common stock, par value	$ 1.00	$ 1.00
Common stock, shares authorized	5,000,000	5,000,000
Common stock, shares issued	1,967,180	1,967,180