Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale Securities, Amortized Cost Basis	$ 7,163,574	$ 5,404,335
Class A Common Stock
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shared authorized	16,000,000	11,000,000
Common stock, shares issued	11,005,220	8,586,058
Common stock, shares outstanding	11,005,220	8,586,058
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	1,032,883
Common stock, shares outstanding	1,005,185	1,032,883
Mortgage Backed Securities, Other [Member]
Available-for-sale Securities, Amortized Cost Basis	3,628,187	2,486,297
Held-to-maturity Securities, Fair Value	$ 544	$ 974