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Regulatory Capital Requirements (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Tier I Capital (to average assets) [Abstract]    
Tier I capital to average assets $ 880,448 $ 810,445
Tier I capital to average assets ratio 9.52% 9.14%
Minimum Tier I leverage capital required for capital adequacy to average assets 4.00% 4.00%
Tier I leverage capital required for classification as well capitalized to average assets 5.00% 5.00%
Common Equity Tier I Capital [Abstract]    
Common equity Tier I capital $ 783,448 $ 713,445
Common equity Tier I capital ratio 10.49% 10.06%
Minimum Tier I capital required for capital adequacy 4.50% 4.50%
Minimum Tier I capital required for capital adequacy plus buffer 6.375% 5.75%
Tier I capital required for classification as well capitalized 6.50% 6.50%
Tier I Capital (to risk weighted assets) [Abstract]    
Tier I capital to risk weighted assets $ 880,448 $ 810,445
Tier I capital to risk weighted assets ratio 11.79% 11.42%
Minimum Tier I capital required for capital adequacy to risk weighted assets 6.00% 6.00%
Minimum Tier I capital required for capital adequacy plus buffer to risk weighted assets 7.875% 7.25%
Tier I capital required for classification as well capitalized to risk weighted assets 8.00% 8.00%
Total Capital (to risk weighted assets) [Abstract]    
Total capital to risk weighted assets $ 954,232 $ 880,874
Total capital to risk weighted assets ratio 12.78% 12.42%
Minimum capital required for capital adequacy to risk weighted assets 8.00% 8.00%
Total capital required for capital adequacy plus buffer to average assets 9.875% 9.25%
Capital required for classification as well capitalized to risk weighted assets 10.00% 10.00%
NBT Bank [Member]    
Tier I Capital (to average assets) [Abstract]    
Tier I capital to average assets $ 825,863 $ 756,521
Tier I capital to average assets ratio 8.98% 8.59%
Minimum Tier I leverage capital required for capital adequacy to average assets 4.00% 4.00%
Tier I leverage capital required for classification as well capitalized to average assets 5.00% 5.00%
Common Equity Tier I Capital [Abstract]    
Common equity Tier I capital $ 825,863 $ 756,521
Common equity Tier I capital ratio 11.14% 10.74%
Minimum Tier I capital required for capital adequacy 4.50% 4.50%
Minimum Tier I capital required for capital adequacy plus buffer 6.375% 5.75%
Tier I capital required for classification as well capitalized 6.50% 6.50%
Tier I Capital (to risk weighted assets) [Abstract]    
Tier I capital to risk weighted assets $ 825,863 $ 756,521
Tier I capital to risk weighted assets ratio 11.14% 10.74%
Minimum Tier I capital required for capital adequacy to risk weighted assets 6.00% 6.00%
Minimum Tier I capital required for capital adequacy plus buffer to risk weighted assets 7.875% 7.25%
Tier I capital required for classification as well capitalized to risk weighted assets 8.00% 8.00%
Total Capital (to risk weighted assets) [Abstract]    
Total capital to risk weighted assets $ 899,647 $ 826,950
Total capital to risk weighted assets ratio 12.14% 11.74%
Minimum capital required for capital adequacy to risk weighted assets 8.00% 8.00%
Total capital required for capital adequacy plus buffer to average assets 9.875% 9.25%
Capital required for classification as well capitalized to risk weighted assets 10.00% 10.00%