SUFFOLK BANCORP (PARENT COMPANY ONLY) CONDENSED FINANCIAL STATEMENTS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets [Abstract]
Other assets	$ 6,465				$ 3,723				$ 6,465	$ 3,723
TOTAL ASSETS	2,092,291				2,168,592				2,092,291	2,168,592
Liabilities and Stockholders' Equity [Abstract]
Other liabilities	11,823				14,888				11,823	14,888
Stockholders' Equity	215,028				197,258				215,028	197,258	$ 182,733
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,092,291				2,168,592				2,092,291	2,168,592
Expense [Abstract]
Other expense									6,660	6,390	5,733
Income before income tax expense									27,453	23,573	19,015
NET INCOME	3,733	$ 5,475	$ 5,785	$ 4,838	3,637	$ 4,923	$ 5,118	$ 4,009	19,831	17,687	15,295
Total Comprehensive Income									18,888	15,954	15,741
Cash Flows From Operating Activities [Abstract]
Net income	3,733	$ 5,475	$ 5,785	4,838	3,637	$ 4,923	$ 5,118	4,009	19,831	17,687	15,295
Stock-based compensation									1,131	949	811
Increase in other assets									2,743	(651)	366
Net cash provided by operating activities									19,822	19,610	11,426
Cash Flows From Investing Activities [Abstract]
Net cash provided by (used in) investing activities									101,770	(234,109)	(263,143)
Cash Flows From Financing Activities [Abstract]
Dividend reinvestment and stock option exercises									201	539	246
Net cash (used in) provided by financing activities									(94,824)	257,069	174,881
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									26,768	42,570	(76,836)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD				98,086				55,516	98,086	55,516	132,352
CASH AND CASH EQUIVALENTS, END OF PERIOD	124,854				98,086				124,854	98,086	55,516
Parent Company [Member]
Assets [Abstract]
Due from banks	89				2,988				89	2,988	1,322
Investment in the Bank	213,599				193,252				213,599	193,252	180,926
Other assets	1,340				1,018				1,340	1,018	674
TOTAL ASSETS	215,028				197,258				215,028	197,258	182,922
Liabilities and Stockholders' Equity [Abstract]
Other liabilities	0				0				0	0	189
Stockholders' Equity	215,028				197,258				215,028	197,258	182,733
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	215,028				197,258				215,028	197,258	182,922
Income [Abstract]
Dividends from the Bank									1,182	3,763	1,399
Other income									0	0	176
Expense [Abstract]
Other expense									41	92	837
Income before income tax expense									1,141	3,671	738
Equity in undistributed earnings of the Bank									18,690	14,016	14,557
NET INCOME									19,831	17,687	15,295
Total Comprehensive Income									18,888	15,954	15,741
Cash Flows From Operating Activities [Abstract]
Net income									19,831	17,687	15,295
Less: equity in undistributed earnings of the Bank									(18,690)	(14,016)	(14,557)
Stock-based compensation									1,131	949	811
Disqualifying dispositions on stock option exercises									0	(43)	0
Increase in other assets									(322)	(344)	(4)
(Decrease) increase in other liabilities									0	(189)	189
Net cash provided by operating activities									1,950	4,044	1,734
Cash Flows From Investing Activities [Abstract]
Advances to the Bank									(2,600)	0	0
Net cash provided by (used in) investing activities									(2,600)	0	0
Cash Flows From Financing Activities [Abstract]
Dividend reinvestment and stock option exercises									2,502	1,385	382
Dividends paid									(4,751)	(3,763)	(1,399)
Net cash (used in) provided by financing activities									(2,249)	(2,378)	(1,017)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									(2,899)	1,666	717
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD				$ 2,988				$ 1,322	2,988	1,322	605
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 89				$ 2,988				$ 89	$ 2,988	$ 1,322