Postretirement Benefits - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) plan	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of nonqualified, noncontributory supplemental benefit pension plans | plan	2
Defined benefit plan contributions	$ 15,800,000	$ 15,500,000	$ 14,600,000
Liability for SERP	$ 74,687,000	67,243,000
Maximum allowed investment percentage in any single issuer in pension plan assets at time of purchase	10.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plans, future	$ 20,000,000
Capped eligible compensation	1,000,000
Supplemental Employee Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Liability for closed supplemental retirement pension plans	3,000,000	3,000,000
Funded Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	411,000,000	371,000,000
Unfunded Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 69,000,000	$ 63,000,000