Postretirement Benefits - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of nonqualified, noncontributory supplemental benefit pension plans | plan	2
Number of active noncontributory supplemental benefit pension plans | plan	1
Defined benefit plan contributions	$ 21,300,000	$ 15,800,000	$ 15,500,000
Maximum allowed investment percentage in any single issuer in pension plan assets at time of purchase	10.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plans, future	$ 30,000,000
Maximum
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plans, future	40,000,000
Capped eligible compensation	1,000,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions	21,272,000	15,757,000
Nonqualified | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	75,000,000	69,000,000
Funded | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 466,000,000	$ 411,000,000