Parent Company Only - Condensed Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Interest and dividend income:
Total interest income	$ 18,801	$ 18,948	$ 19,114	$ 18,705	$ 18,607	$ 18,278	$ 18,182	$ 18,234	$ 75,568	$ 73,301
Noninterest expense:
Salaries									22,580	22,159
Other general and administrative expenses									4,555	4,423
Total noninterest expense	9,320	9,401	9,511	9,774	9,224	9,503	9,374	9,393	38,006	37,494
Income before income taxes	6,193	7,141	6,475	8,497	6,380	6,054	6,310	6,579	28,306	25,323
Income taxes	1,148	1,775	1,415	1,973	1,737	1,268	1,347	1,759	6,311	6,111
Net income	$ 5,045	$ 5,366	$ 5,060	$ 6,524	$ 4,643	$ 4,786	$ 4,963	$ 4,820	21,995	19,212
Territorial Bancorp Inc.
Interest and dividend income:
Dividends from Territorial Savings Bank									21,250	14,500
Interest-earning deposit with Territorial Savings Bank									35	43
Total interest income									21,285	14,543
Noninterest expense:
Salaries									43	38
Other general and administrative expenses									676	791
Total noninterest expense									719	829
Income before income taxes									20,566	13,714
Income taxes									(177)	(107)
Income before equity in undistributed earnings in subsidiaries									20,743	13,821
Equity in undistributed earnings of Territorial Savings Bank, net of dividends									1,252	5,391
Net income									$ 21,995	$ 19,212