Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized cost	$ 345,709	$ 336,751
Held-to-maturity securities, fair value	2,074,443	1,991,421
Equity securities, amortized cost	$ 1,635	$ 1,635
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	3,610,329	3,608,329
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	1,957,580	1,959,580