Securities, Exposure to Bond Issuers that Exceeded 10% of Stockholders' Equity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	$ 438,399	$ 501,656
Stockholders' Equity, Total [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	333,643
Fair value	317,333
Stockholders' Equity, Total [Member] | U.S. Treasuries [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	16,597
Fair value	16,595
Stockholders' Equity, Total [Member] | Federal Home Loan Bank (FHLB) [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	48,397
Fair value	45,709
Stockholders' Equity, Total [Member] | Federal Home Loan Mortgage Corporation (Freddie Mac-FHLMC) [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	49,086
Fair value	47,200
Stockholders' Equity, Total [Member] | Federal National Mortgage Association (Fannie Mae-FNMA) [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	102,289
Fair value	97,094
Stockholders' Equity, Total [Member] | Federal Farm Credit Bank (FFCB) [Member]
Exposure to investment securities issuers that exceeded 10% of shareholders' equity [Abstract]
Amortized cost	117,274
Fair value	$ 110,735