Contingent Liabilities - Narrative (Detail)		1 Months Ended			12 Months Ended
	Jan. 14, 2014 case	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Jan. 31, 2015 trust	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2017 USD ($) shares
Loss Contingencies [Line Items]
Accruals recorded for pending litigation or threatened legal actions or regulatory matters							$ 0
Number of opt-out cases filed by merchants | case	30
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain		$ 123,100,000	$ 99,900,000		$ 123,100,000	$ 99,900,000
Common stock issued (in shares) | shares		1,100,000	1,000,000		1,100,000.0	1,000,000.0
Visa class b common stock shares held (in shares) | shares							4,100,000
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							$ 30,000,000