Employee Benefits and Incentive Plans - Qualified Retirement Plan and Non-Qualified Retirement Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Retirement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees age limit	18 years	18 years
Employees minimum working hours	1000 hours	1000 hours
Employees Contribution Percentage	50.00%	50.00%
Match employee contribution up to a maximum of their base salary	14.00%	14.00%
Percentage of amounts in notional equity shares, vesting in each year	25.00%	25.00%
Attainment of vested employees salary contribution	100.00%	100.00%
Vested maturity period	5 years	5 years
Total employer contribution under the plan	$ 8.5		$ 7.4	$ 6.9
Pension Plans, Defined Benefit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees age limit	22 years	22 years
Match employee contribution up to a maximum of their base salary	2.00%	2.00%
Total employer contribution under the plan	$ 1.2		1.3	1.0
Minimum employee earnings | £		£ 10,000
Employees successful probationary completion period	3 months	3 months
Deferred Profit Sharing Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees Contribution Percentage	50.00%	50.00%
Match employee contribution up to a maximum of their base salary	3.00%	3.00%
Attainment of vested employees salary contribution	100.00%	100.00%
Total employer contribution under the plan	$ 0.3		0.3	0.2
Non Qualified Retirement Savings Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees Contribution Percentage	50.00%	50.00%
Attainment of vested employees salary contribution	100.00%	100.00%
Vested maturity period	3 years	3 years
Total employer contribution under the plan	$ 2.6		1.9	$ 1.9
Assets Under Trust	$ 46.9		$ 47.5
Non Qualified Deferred Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees Contribution Percentage	50.00%	50.00%	50.00%	50.00%
Percentage of amounts in notional equity shares, vesting in each year	20.00%	20.00%	20.00%	20.00%
Attainment of vested employees salary contribution	100.00%	100.00%
Vested maturity period	5 years	5 years
Total employer contribution under the plan	$ 1.7		$ 1.1	$ 0.9
Assets Under Trust	9.2		9.3
Non Qualified Deferred Compensation Plan [Member] | Notional [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notional equity shares in the deferred Compensation Plan	$ 28.4		$ 46.3