Employee Benefit Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Rate	Dec. 31, 2018 GBP (£) Rate	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($)
Pension and Other Postretirement Benefits Disclosure
Health Care Cost Trend Rate Assumed for Next Fiscal Year | Rate	7.00%		7.00%
Ultimate Health Care Cost Trend Rate | Rate	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2024	2024
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	$ 600,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(500,000)
Future Amortization of Actuarial Gain (Loss)	18,400,000
Future Amortization of Prior Service Cost (Credit)	200,000
Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Plan Amendments	0		$ 0
Investment Level 3 Transfers Into	0		0
Investment Level 3 Transfers Out of	$ 0		$ 0
Interest Rate During Period	4.73%	4.73%	4.76%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	5.75%	5.75%	5.75%
Domestic Plan | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Plan Amendments	$ 0		$ 7,600,000
Average Remaining Life Expectancy of Plan Participants	24 years	24 years
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.75%	6.75%	7.25%
Retirement Benefits, Description	We had no regulatory contribution requirements for our U.S. qualified defined benefit plan in 2018	We had no regulatory contribution requirements for our U.S. qualified defined benefit plan in 2018
Domestic Plan | Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Matching Contribution Percent of Employees' Gross Pay | Rate	5.00%	5.00%
Employer Discretionary Contribution Percentage of earnings | Rate	4.50%	4.50%
Cost Recognized	$ 72,700,000.0		$ 78,400,000.0	$ 72,200,000.0
Domestic Plan | Equity Securities | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	36.00%
Domestic Plan | Fixed Maturity Securities | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	40.00%
Domestic Plan | Alternative Investments | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	24.00%
Domestic Plan | Minimum | Private Equity Funds of Funds | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	5 years	5 years
Domestic Plan | Maximum | Private Equity Funds of Funds | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	8 years	8 years
Foreign Plan | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Plan Amendments	$ 300,000		$ 0
Average Remaining Life Expectancy of Plan Participants	31 years	31 years
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	3.90%	3.90%	3.90%
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%	5.00%
Employer Discretionary Contribution Percentage of earnings	6.00%	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%	1.00%
Cost Recognized	$ 4,200,000		$ 4,200,000	$ 4,900,000
Foreign Plan | Interest Rate Contract | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%
Foreign Plan | Diversified Growth Assets | Pension Plan
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	65.00%