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ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
May 12, 2017 | Feb. 03, 2017 | Jan. 04, 2016 | Dec. 04, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | $ 107,414 | $ 575 | $ (25,505)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 734,430 | $ 465,142 | $ 463,252
NRS [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1] |  | $ 70,073 |  |  |  |  |
Community Bank System, Inc. common stock |  |  | 78,483 |  |  |  |  |
Total net consideration paid |  |  | 148,556 |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  | 11,063 |  |  |  |  |
Investment securities |  |  | 20,294 |  |  |  |  |
Loans |  |  | 0 |  |  |  |  |
Premises and equipment |  |  | 411 |  |  |  |  |
Accrued interest receivable |  |  | 72 |  |  |  |  |
Other assets |  |  | 8,088 |  |  |  |  |
Deposits |  |  | 0 |  |  |  |  |
Other liabilities |  |  | (28,002) |  |  |  |  |
Short-term advances |  |  | 0 |  |  |  |  |
Securities sold under agreement to repurchase, short-term |  |  | 0 |  |  |  |  |
Long-term debt |  |  | 0 |  |  |  |  |
Subordinated debt held by unconsolidated subsidiary trusts |  |  | 0 |  |  |  |  |
Total identifiable assets, net |  |  | 72,126 |  |  |  |  |
Goodwill |  |  | 76,430 |  |  |  |  |
NRS [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  | 0 |  |  |  |  |
NRS [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  | $ 60,200 |  |  |  |  |
Merchants Bancshares, Inc. [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1] | $ 82,898 |  |  |  |  |  |
Community Bank System, Inc. common stock |  | 262,254 |  |  |  |  |  |
Total net consideration paid |  | 345,152 |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  | 40,730 |  |  |  |  |  |
Investment securities |  | 370,648 |  |  |  |  |  |
Loans |  | 1,488,157 |  |  |  |  |  |
Premises and equipment |  | 16,608 |  |  |  |  |  |
Accrued interest receivable |  | 4,773 |  |  |  |  |  |
Other assets |  | 51,585 |  |  |  |  |  |
Deposits |  | (1,448,406) |  |  |  |  |  |
Other liabilities |  | (11,750) |  |  |  |  |  |
Short-term advances |  | (80,000) |  |  |  |  |  |
Securities sold under agreement to repurchase, short-term |  | (278,076) |  |  |  |  |  |
Long-term debt |  | (3,615) |  |  |  |  |  |
Subordinated debt held by unconsolidated subsidiary trusts |  | (20,619) |  |  |  |  |  |
Total identifiable assets, net |  | 156,106 |  |  |  |  |  |
Goodwill |  | 189,046 |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  | 23,214 |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  | $ 2,857 |  |  |  |  |  |
WJL Agencies Inc [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1] |  |  | $ 575 |  |  |  |
Community Bank System, Inc. common stock |  |  |  | 0 |  |  |  |
Total net consideration paid |  |  |  | 575 |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  |  | 0 |  |  |  |
Investment securities |  |  |  | 0 |  |  |  |
Loans |  |  |  | 0 |  |  |  |
Premises and equipment |  |  |  | 0 |  |  |  |
Accrued interest receivable |  |  |  | 0 |  |  |  |
Other assets |  |  |  | 0 |  |  |  |
Deposits |  |  |  | 0 |  |  |  |
Other liabilities |  |  |  | 0 |  |  |  |
Short-term advances |  |  |  | 0 |  |  |  |
Securities sold under agreement to repurchase, short-term |  |  |  | 0 |  |  |  |
Long-term debt |  |  |  | 0 |  |  |  |
Subordinated debt held by unconsolidated subsidiary trusts |  |  |  | 0 |  |  |  |
Total identifiable assets, net |  |  |  | 288 |  |  |  |
Goodwill |  |  |  | 287 |  |  |  |
WJL Agencies Inc [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  | 0 |  |  |  |
WJL Agencies Inc [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 288 |  |  |  |
Other [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1],[2] |  |  |  |  | 6,775 |  |
Community Bank System, Inc. common stock | [2] |  |  |  |  | 2,395 |  |
Total net consideration paid | [2] |  |  |  |  | 9,170 |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents | [2] |  |  |  |  | 339 |  |
Investment securities | [2] |  |  |  |  | 0 |  |
Loans | [2] |  |  |  |  | 0 |  |
Premises and equipment | [2] |  |  |  |  | 27 |  |
Accrued interest receivable | [2] |  |  |  |  | 0 |  |
Other assets | [2] |  |  |  |  | 583 |  |
Deposits | [2] |  |  |  |  | 0 |  |
Other liabilities | [2] |  |  |  |  | (1,217) |  |
Short-term advances | [2] |  |  |  |  | 0 |  |
Securities sold under agreement to repurchase, short-term | [2] |  |  |  |  | 0 |  |
Long-term debt | [2] |  |  |  |  | 0 |  |
Subordinated debt held by unconsolidated subsidiary trusts | [2] |  |  |  |  | 0 |  |
Total identifiable assets, net | [2] |  |  |  |  | 5,358 |  |
Goodwill | [2] |  |  |  |  | 3,812 |  |
Other [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles | [2] |  |  |  |  | 0 |  |
Other [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles | [2] |  |  |  |  | 5,626 |  |
Total [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1] |  |  |  |  | 159,746 |  |
Community Bank System, Inc. common stock |  |  |  |  |  | 343,132 |  |
Total net consideration paid |  |  |  |  |  | 502,878 |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  |  |  |  | 52,132 |  |
Investment securities |  |  |  |  |  | 390,942 |  |
Loans |  |  |  |  |  | 1,488,157 |  |
Premises and equipment |  |  |  |  |  | 17,046 |  |
Accrued interest receivable |  |  |  |  |  | 4,845 |  |
Other assets |  |  |  |  |  | 60,256 |  |
Deposits |  |  |  |  |  | (1,448,406) |  |
Other liabilities |  |  |  |  |  | (40,969) |  |
Short-term advances |  |  |  |  |  | (80,000) |  |
Securities sold under agreement to repurchase, short-term |  |  |  |  |  | (278,076) |  |
Long-term debt |  |  |  |  |  | (3,615) |  |
Subordinated debt held by unconsolidated subsidiary trusts |  |  |  |  |  | (20,619) |  |
Total identifiable assets, net |  |  |  |  |  | 233,590 |  |
Goodwill |  |  |  |  |  | 269,288 |  |
Total [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | 23,214 |  |
Total [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | $ 68,683 |  |
Oneida Financial Corp [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | [1] |  |  |  | $ 56,266 |  |  |
Community Bank System, Inc. common stock |  |  |  |  | 102,202 |  |  |
Total net consideration paid |  |  |  |  | 158,468 |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  |  |  | 81,772 |  |  |
Investment securities |  |  |  |  | 225,729 |  |  |
Loans |  |  |  |  | 399,422 |  |  |
Premises and equipment |  |  |  |  | 22,212 |  |  |
Accrued interest receivable |  |  |  |  | 1,133 |  |  |
Other assets |  |  |  |  | 26,529 |  |  |
Deposits |  |  |  |  | (699,241) |  |  |
Other liabilities |  |  |  |  | (1,333) |  |  |
Short-term advances |  |  |  |  | 0 |  |  |
Securities sold under agreement to repurchase, short-term |  |  |  |  | 0 |  |  |
Long-term debt |  |  |  |  | 0 |  |  |
Subordinated debt held by unconsolidated subsidiary trusts |  |  |  |  | 0 |  |  |
Total identifiable assets, net |  |  |  |  | 68,787 |  |  |
Goodwill |  |  |  |  | 89,681 |  |  |
Oneida Financial Corp [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  |  | 2,570 |  |  |
Oneida Financial Corp [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  |  | $ 9,994 |  |  |
[1] | Includes NECM $0.2 million contingent cash payment consideration.
[2] | Includes amounts related to the BAS, Dryfoos, NECM and GBR acquisitions.

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