Off-Balance Sheet Activities and Mortgage Banking - Derivatives Designated as Hedging Instruments (Details) - Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Mortgage Banking
Assets, Fair Value	$ 362	$ 246
Liabilities, Fair Value	43	20
Derivative loan commitments
Mortgage Banking
Assets, Fair Value	352	211
Derivative loan commitments hedged with best efforts \| Other assets
Mortgage Banking
Assets, Fair Value	111	32
Derivative loan commitments hedged with TBAs \| Other assets
Mortgage Banking
Assets, Fair Value	241	179
Forward loan sale commitments
Mortgage Banking
Assets, Fair Value	10	35
Liabilities, Fair Value	43	20
Best efforts contracts hedging
Mortgage Banking
Assets, Fair Value	6	25
Best efforts contracts - hedging commitments \| Other assets
Mortgage Banking
Assets, Fair Value	4	7
Best efforts contracts - hedging loans held for sale \| Other assets
Mortgage Banking
Assets, Fair Value	2	18
Mandatory delivery contracts \| Other assets
Mortgage Banking
Assets, Fair Value	4	10
TBA securities \| Other liabilities
Mortgage Banking
Liabilities, Fair Value	$ 43	$ 20

Coastway Bancorp, Inc. 10-Q