Employee Benefit Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($) Rate	Dec. 31, 2015 USD ($)
Pension and Other Postretirement Benefits Disclosure
Health Care Cost Trend Rate Assumed for Next Fiscal Year | Rate	7.00%	7.00%	7.00%	7.00%		7.00%
Ultimate Health Care Cost Trend Rate | Rate	5.00%	5.00%	5.00%	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2024
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation			$ 700,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation			(700,000)
Future Amortization of Actuarial Gain (Loss)	$ 22,300,000	$ 22,300,000	22,300,000	$ 22,300,000
Future Amortization of Prior Service Cost (Credit)	$ 200,000	$ 200,000	200,000	$ 200,000
Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Plan Amendments			0			$ (3,700,000)
Level 3 Transfers Into			0			0
Level 3 Transfers Out of			0			$ 0
Expected Long-term Return on Assets Assumptions Used Calculating Net Periodic Benefit Cost		5.75%		5.75%		5.75%
Interest Rate During Period		4.76%				4.99%
Minimum
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	1 year
Maximum
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	10 years
Domestic Plan | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Plan Amendments			7,600,000			$ 0	$ (7,500,000)
Average Remaining Life Expectancy of Plan Participants	25 years
Expected Long-term Return on Assets Assumptions Used Calculating Net Periodic Benefit Cost		7.25%				7.50%
Retirement Benefits, Description	We had no regulatory contribution requirements for our U.S. qualified defined benefit plan in 2017
Domestic Plan | Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Matching Contribution Percent of Employees' Gross Pay | Rate				5.00%
Employer Discretionary Contribution Percentage of earnings | Rate				4.50%
Cost Recognized			78,400,000.0			$ 72,200,000.0	71,500,000.0
Domestic Plan | Equity Securities | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Target Allocation Percentage	0.36
Domestic Plan | Fixed Maturity Securities | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Target Allocation Percentage	.4
Domestic Plan | Alternative Investments | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Target Allocation Percentage	0.24
Domestic Plan | Minimum | Private Equity Funds | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	5 years
Domestic Plan | Maximum | Private Equity Funds | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	8 years
Foreign Plan | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Plan Amendments			0			$ 0
Average Remaining Life Expectancy of Plan Participants	32 years
Expected Long-term Return on Assets Assumptions Used Calculating Net Periodic Benefit Cost		3.90%				4.90%
Payment for Pension Benefits			0
Foreign Plan | Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Matching Contribution Percent of Employees' Gross Pay		5.00%
Employer Discretionary Contribution Percentage of earnings		6.00%
Employer Matching Contribution for Every Pound on the First One Percent of Employee Contributions | £					£ 2
Employee Contribution Percentage Received 2-for-1 Employer Match		1.00%
Cost Recognized			$ 4,200,000.0			$ 4,900,000.0	$ 5,700,000.0
Foreign Plan | Diversified Growth Assets | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Target Allocation Percentage	0.65
Foreign Plan | Interest Rate Contract | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Target Allocation Percentage	0.35