Debt Securities Held-to-Maturity - Unrealized Losses and Estimated Fair Value (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Unrealized losses
Less than 12 months	$ 0	$ 34,000
12 months or more	5,000	222,000
Total	5,000	256,000
Estimated fair value
Less than 12 months	0	2,133,000
12 months or more	378,000	7,116,000
Total	378,000	9,249,000
Other-than-temporary impairment charges on securities held-to-maturity	0	0	$ 0
Mortgage-Backed Securities, Pass-Through Certificates
Unrealized losses
Less than 12 months	0	34,000
12 months or more	5,000	222,000
Total	5,000	256,000
Estimated fair value
Less than 12 months	0	2,133,000
12 months or more	378,000	7,116,000
Total	$ 378,000	$ 9,249,000
Number of held-to-maturity securities in an unrealized loss position, greater than twelve months | security	1