Average Remaining Life Expectancy of Plan Participants

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Defined Contribution Plan, Employee Contribution Percentage Received 2-for-1 Employer Match

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Defined Contribution Plan, Employer Discretionary Contribution Percentage of Earnings

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Defined Contribution Plan, Employer Matching Contribution for Every Pound on the First One Percent of Employee Contributions

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- Definition

Pension and Other Postretirement Benefits Disclosure [Line Items]

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- Definition

Private Equity Partnership Future Liquidation Term

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- Definition

The average effective interest rate during the reporting period.

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- Definition

Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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-Name Accounting Standards Codification
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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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-Publisher FASB
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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

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Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

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- Definition

Amount of cost for defined contribution plan.

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-SubTopic 70
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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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No definition available.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Description of nature and type of retirement benefits.

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Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Pension and Other Postretirement Benefits Disclosure
Health Care Cost Trend Rate Assumed for Next Fiscal Year	6.50%	6.50%	6.50%
Ultimate Health Care Cost Trend Rate	5.00%	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2027	2027
Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 126.1		$ 146.3
Average Remaining Life Expectancy of Plan Participants	25 years	25 years
Retirement Benefits, Description	We had no regulatory contribution requirements for our U.S. qualified defined benefit plan in 2020	We had no regulatory contribution requirements for our U.S. qualified defined benefit plan in 2020
Pension Plan \| UNITED STATES \| Group Annuity Contact
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 44.0		59.5
Pension Plan \| UNITED KINGDOM
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 5.1		4.8
Average Remaining Life Expectancy of Plan Participants	31 years	31 years
Payment for Pension Benefits	$ 0.0
Other Postretirement Benefit Plans
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Benefit Obligation, Benefits Paid	10.6		11.2
Investment Level 3 Transfers Into	0.0		0.0
Investment Level 3 Transfers Out of	$ 0.0		$ 0.0
Interest Rate During Period	4.87%	4.87%	4.74%
Other Postretirement Benefit Plans \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Matching Contribution Percent of Employees' Gross Pay	5.00%	5.00%
Employer Discretionary Contribution Percentage of earnings	4.50%	4.50%
Cost Recognized	$ 83.4		$ 77.3	$ 72.7
Other Postretirement Benefit Plans \| UNITED KINGDOM
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	$ 5.0		$ 4.4	$ 4.2
Equity Securities \| Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	36.00%	36.00%
Fixed Maturity Securities \| Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	40.00%	40.00%
Alternative Investments \| Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	24.00%	24.00%
Diversified Growth Assets \| Pension Plan \| UNITED KINGDOM
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	65.00%	65.00%
Interest Rate Contract \| Pension Plan \| UNITED KINGDOM
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%	35.00%
Minimum \| Private Equity Funds of Funds \| Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	5 years	5 years
Maximum \| Private Equity Funds of Funds \| Pension Plan \| UNITED STATES
Pension and Other Postretirement Benefits Disclosure
Private Equity Partnership Future Liquidation Term	8 years	8 years