Contingent Liabilities - Narrative (Detail) shares in Millions		1 Months Ended		12 Months Ended
	Jun. 28, 2018 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 trust	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Accruals recorded for pending litigation or threatened legal actions or regulatory matters						$ 0
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain		$ 123,100,000	$ 99,900,000	$ 123,100,000
Common stock issued (in shares) | shares		1.1	1.0	1.1
Visa class b common stock shares held (in shares) | shares						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 served 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						$ 20,000,000
Visa Class B Litigation Case | Visa Inc.
Loss Contingencies [Line Items]
Payment to escrow deposit	$ 600,000,000