Employee Benefits - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Investment year	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of investments held in diversified portfolios | Investment	2
Postretirement Health Care, minimum retirement age | year	55
Postretirement Health Care, minimum service term	15 years
Estimated contribution defined contribution plans	$ 50,000,000	$ 46,800,000	$ 46,800,000
Trust for Benefit of Employees
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	106,900,000	106,100,000
Contributions made to the pension plan	8,500,000	16,500,000
Supplemental Plan
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 0	$ 0
Prior service costs, amortization period (years)	10 years
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.46%	4.71%
Other Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.46%	4.71%
Ultimate health care rate	5.00%
Other Postretirement Plan | Pre-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Current health care cost trend rate	7.00%
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]
Current health care cost trend rate	7.00%
Year in which ultimate trend rate will be reached	2026
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 1,393,500,000	$ 1,342,000,000
Amortization from AOCI of the net actuarial gain (loss)	25,900,000
Amortization from AOCI of the prior service benefit	$ (300,000)
Pension Plans, Defined Benefit | Alternative Investment
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	20.00%
Pension Plans, Defined Benefit | U.S. | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	20.00%
Pension Plans, Defined Benefit | Non U.S. | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	15.00%
Pension Plans, Defined Benefit | Government Sponsored Agency | Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	45.00%
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 35,700,000	47,500,000
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds | Alternative Investment
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 64,800,000	63,400,000
Pension Plans, Defined Benefit | Northern Trust Hedge Funds | Alternative Investment
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Pension Plans, Defined Benefit | Real Estate | Alternative Investment
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Pension Plans, Defined Benefit | Global Listed Infrastructure | Alternative Investment
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Pension Plans, Defined Benefit | Derivative Financial Instruments, Assets
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 0	0
U.S. Plan
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	1,393,500,000	1,342,000,000	$ 1,440,800,000
Contributions made to the pension plan	$ 0	$ 0
Prior service costs, amortization period (years)	11 years
Expected long-term rate of return on plan assets	6.75%
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.46%	4.71%
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	$ 225,000,000
U.S. Plan | Northern Trust Private Equity Funds | Upper Limit
Defined Benefit Plan Disclosure [Line Items]
Equity target allocation of plan assets	20.00%