Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized	10,000,000	10,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Available-for-sale Securities, Amortized Cost Basis	$ 7,079,287	$ 6,885,797
Class A Common Stock
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shared authorized	16,000,000	16,000,000
Common stock, shares issued	11,005,220	11,005,220
Common stock, shares outstanding	11,005,220	11,005,220
Class B Common Stock
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shared authorized	2,000,000	2,000,000
Common stock, shares issued	1,005,185	1,005,185
Common stock, shares outstanding	1,005,185	1,005,185
Mortgage Backed Securities, Other
Available-for-sale Securities, Amortized Cost Basis	$ 5,259,466	$ 4,692,447
Held-to-maturity Securities, Fair Value	$ 104	$ 265