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ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands |  |  |  |  |  | 6 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
May 12, 2017 | Mar. 01, 2017 | Feb. 03, 2017 | Jan. 04, 2017 | Jan. 04, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | $ 105,402 | $ 575 |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 733,604 | $ 465,142 | $ 465,142
BAS [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  |  |  | $ 1,200 |  |  |  |
Community Bank System, Inc. common stock |  |  |  | 0 |  |  |  |
Total net consideration paid |  |  |  | 1,200 |  |  |  |
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 |  |  |  | 1,200 |  |  |  |
Goodwill |  |  |  | 0 |  |  |  |
BAS [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  | 0 |  |  |  |
BAS [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 1,200 |  |  |  |
NRS [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  |  | $ 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 |  |  | (28,106) |  |  |  |  |
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 |  |  | 72,022 |  |  |  |  |
Goodwill |  |  | 76,534 |  |  |  |  |
NRS [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  | 0 |  |  |  |  |
NRS [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  |  | $ 60,200 |  |  |  |  |
Dryfoos [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  | $ 3,024 |  |  |  |  |  |
Community Bank System, Inc. common stock |  | 0 |  |  |  |  |  |
Total net consideration paid |  | 3,024 |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  | 0 |  |  |  |  |  |
Investment securities |  | 0 |  |  |  |  |  |
Loans |  | 0 |  |  |  |  |  |
Premises and equipment |  | 27 |  |  |  |  |  |
Accrued interest receivable |  | 0 |  |  |  |  |  |
Other assets |  | 272 |  |  |  |  |  |
Deposits |  | 0 |  |  |  |  |  |
Other liabilities |  | (582) |  |  |  |  |  |
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 |  | 1,374 |  |  |  |  |  |
Goodwill |  | 1,650 |  |  |  |  |  |
Dryfoos [Member] | Core Deposits [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  | 0 |  |  |  |  |  |
Dryfoos [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Intangibles |  | $ 1,657 |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash | $ 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 | 370,480 |  |  |  |  |  |  |
Loans | 1,489,273 |  |  |  |  |  |  |
Premises and equipment | 12,973 |  |  |  |  |  |  |
Accrued interest receivable | 4,773 |  |  |  |  |  |  |
Other assets | 46,363 |  |  |  |  |  |  |
Deposits | (1,448,408) |  |  |  |  |  |  |
Other liabilities | (5,071) |  |  |  |  |  |  |
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 | 154,874 |  |  |  |  |  |  |
Goodwill | 190,278 |  |  |  |  |  |  |
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 |  |  |  |  | $ 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 |  |  |
Total [Member] |  |  |  |  |  |  |  |
Consideration paid [Abstract] |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | 157,195 |  |
Community Bank System, Inc. common stock |  |  |  |  |  | 340,737 |  |
Total net consideration paid |  |  |  |  |  | 497,932 |  |
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] |  |  |  |  |  |  |  |
Cash and cash equivalents |  |  |  |  |  | 51,793 |  |
Investment securities |  |  |  |  |  | 390,774 |  |
Loans |  |  |  |  |  | 1,489,273 |  |
Premises and equipment |  |  |  |  |  | 13,411 |  |
Accrued interest receivable |  |  |  |  |  | 4,845 |  |
Other assets |  |  |  |  |  | 54,723 |  |
Deposits |  |  |  |  |  | (1,448,408) |  |
Other liabilities |  |  |  |  |  | (33,759) |  |
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 |  |  |  |  |  | 229,470 |  |
Goodwill |  |  |  |  |  | 268,462 |  |
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 |  |  |  |  |  | $ 65,914 |  |

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