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Regulatory Capital Requirements (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Tier I Capital (to average assets) [Abstract]    
Tier I capital to average assets $ 810,445 $ 773,111
Tier I capital to average assets ratio 9.14% 9.11%
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 $ 713,445 $ 676,111
Common equity Tier I capital ratio 10.06% 9.98%
Minimum Tier I capital required for capital adequacy 4.50% 4.50%
Minimum Tier I capital required for capital adequacy plus buffer 5.75% 5.125%
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 $ 810,445 $ 773,111
Tier I capital to risk weighted assets ratio 11.42% 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 7.25% 6.625%
Tier I capital required for classification as well capitalized to risk weighted assets 6.00% 8.00%
Total Capital (to risk weighted assets) [Abstract]    
Total capital to risk weighted assets $ 880,874 $ 839,152
Total capital to risk weighted assets ratio 12.42% 12.39%
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 9.25% 8.625%
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 $ 756,521 $ 723,992
Tier I capital to average assets ratio 8.59% 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 $ 756,521 $ 723,992
Common equity Tier I capital ratio 10.74% 10.76%
Minimum Tier I capital required for capital adequacy 4.50% 4.50%
Minimum Tier I capital required for capital adequacy plus buffer 5.75% 5.125%
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 $ 756,521 $ 723,992
Tier I capital to risk weighted assets ratio 10.74% 10.76%
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 7.25% 6.625%
Tier I capital required for classification as well capitalized to risk weighted assets 6.00% 8.00%
Total Capital (to risk weighted assets) [Abstract]    
Total capital to risk weighted assets $ 826,950 $ 790,034
Total capital to risk weighted assets ratio 11.74% 11.75%
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 9.25% 8.625%
Capital required for classification as well capitalized to risk weighted assets 10.00% 10.00%