Commitments and Contingent Liabilities (Narrative) (Details) shares in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2023 USD ($) shares	Dec. 31, 2015 trust shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2016 shares
Loss Contingencies [Line Items]
Minimum securities lending related collateral percentage			100.00%
Commitments and letters of credit			$ 213,097,800,000		$ 179,758,000,000
Resell agreements, period end amounts excluding effects of agreements reported net by counterparty			4,800,000,000		496,800,000
Repurchase agreement counterparty, amount at risk			0		$ 0
Loss contingency accrual			0
Escrow deposit			$ 2,100,000,000
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain	$ 123,100,000	$ 99,900,000
Shares issued (in shares) | shares				1.0		1.1
Common stock shares held (in shares) | shares			4.1		4.1
Remaining Visa shares held by Northern Trust, original cost basis			$ 0		$ 0
Northern Trust Fiduciary Services (Guernsey) Limited | Subsidiaries
Loss Contingencies [Line Items]
Number of administrated trusts serving as trustee on | trust				2
Lower Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual			0
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual			25,000,000
Indemnification Agreement
Loss Contingencies [Line Items]
Credit loss liability			$ 0		$ 0