Postretirement Benefits - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) plan	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of nonqualified, noncontributory supplemental benefit pension plans | plan	2
Defined benefit plan contributions	$ 15,500,000	$ 14,600,000	$ 10,300,000
Insurance premium paid for supplemental pension plan	10,100,000	2,200,000	2,900,000
Insurance cash value of supplemental pension plan	34,000,000	24,000,000
Cash deposited into Rabbi Trust	0	0	$ 6,000,000
Insurance cash value and Rabbi Trust investments supporting supplemental pension plan liability	79,000,000	74,000,000
Unqualified supplemental retirement pension plan liability	67,000,000	71,000,000
Liability for closed supplemental retirement pension plans	$ 3,000,000	3,000,000
Maximum allowed investment percentage in any single issuer in pension plan assets at time of purchase	10.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plans, future	$ 20,000,000
Capped eligible compensation	1,000,000
Funded Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	371,000,000	374,000,000
Unfunded Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 63,000,000	$ 63,000,000