Employee Benefits - Narrative (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Investment Age derivative_instruments	Dec. 31, 2016 USD ($) derivative_instruments	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return on Assets	6.00%
Number of investments held in diversified portfolios | Investment	2
Number of derivatives held by the plan | derivative_instruments	5	0
Postretirement Health Care, minimum retirement age | Age	55
Postretirement Health Care, minimum service term	15 years
Amortization from AOCI included in net periodic postretirement (benefit) expense, next fiscal year	$ 100,000
Estimated contribution defined contribution plans	$ 53,400,000	$ 50,000,000	$ 46,800,000
International Equities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Trust for Benefit of Employees
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 116,700,000	106,900,000
Contributions made to the pension plan	$ 11,500,000	8,500,000
Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	2.00%
Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Global Listed Infrastructure
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Emerging Market Debt
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	4.00%
Long Credit Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	45.00%
Long Government Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	10.00%
Custom Completion
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	10.00%
Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	8.00%
Emerging Markets Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	4.00%
Supplemental Plan
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 0	$ 0
Prior service costs, amortization period (years)	9 years
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.79%	4.46%
Other Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.79%	4.46%
Current health care cost trend rate	8.75%
Ultimate health care rate	4.50%
Other Postretirement Plan | Pre-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Current health care cost trend rate	6.80%
Other Postretirement Plan | Post-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Current health care cost trend rate	6.80%
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 1,506,400,000	$ 1,393,500,000
Amortization from AOCI of the net actuarial gain (loss)	36,500,000
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	29,300,000	35,700,000
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	44,600,000	64,800,000
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 38,600,000	52,100,000
Pension Plans, Defined Benefit | Derivative Financial Instruments, Assets
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan		$ 0
Other Postretirement Plan | Pre-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Year in which ultimate trend rate will be reached	2026
Other Postretirement Plan | Post-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Year in which ultimate trend rate will be reached	2026
U.S. Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service costs, amortization period (years)	11 years
U.S. Plan | Qualified Defined Benefit Pension Plans | Lower Limit
Defined Benefit Plan Disclosure [Line Items]
Contribution to plan assets	$ 0
U.S. Plan | Qualified Defined Benefit Pension Plans | Upper Limit
Defined Benefit Plan Disclosure [Line Items]
Contribution to plan assets	$ 250,000,000
U.S. Plan | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return on Assets	6.75%	7.00%	7.25%
Assets related to the pension plan	$ 1,506,400,000	$ 1,393,500,000	$ 1,342,000,000
Contributions made to the pension plan	$ 0	$ 0
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.79%	4.46%