Investments - Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, All Other Investments [Abstract]
Federal Home Loan Bank stock, at cost	$ 54,019,000	$ 45,021,000
Federal Reserve Bank stock, at cost	37,635,000	37,623,000
Total investments	91,654,000	82,644,000
Impairment charges on investments	$ 0	$ 0	$ 0
Yield on investments, no stated maturity (as a percent)	4.34%	2.96%