Summary of Significant Accounting Policies - Additional Information (Detail) AUD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) Country Segment shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) Contract	Dec. 31, 2014 AUD	Sep. 01, 2013
Significant Accounting Policies [Line Items]
Number of countries in which the company has operations | Country									31
Number of countries in which the company does business through a network of correspondent brokers and consultants | Country									150
Number of reportable operating segments | Segment									3
Percentage of ownership interest	50.00%								50.00%				99.00%
Percentage of variable ownership interest									50.00%
Total revenues	$ 1,334,900,000	$ 1,454,800,000	$ 1,371,400,000	$ 1,231,300,000	$ 1,245,400,000	$ 1,286,800,000	$ 1,179,300,000	$ 915,000,000	$ 5,392,400,000	$ 4,626,500,000	$ 3,179,600,000
Fee revenue generation period, years									1 year
Allowances for estimated policy cancellations									$ 7,400,000	6,800,000
Allowance for doubtful accounts	13,300,000				10,700,000				$ 13,300,000	10,700,000
Maturity period of short-term investments									90 days or less
Gain on foreign currency derivative investment contract										1,900,000	$ 2,600,000
Number of foreign currency derivative investment contracts | Contract											3
Foreign currency derivative investment contract | AUD												AUD 400.0
Premium financing contracts, terms									Premium financing contracts are structured to minimize potential bad debt expense to us. Such receivables are generally considered delinquent after seven days of the payment due date. In normal course, insurance policies are cancelled within one month of the contractual payment due date if the payment remains delinquent.
Outstanding loan receivable	$ 220,200,000				$ 232,600,000				$ 220,200,000	232,600,000
Write-off of amortizable intangible assets									$ 11,500,000	1,800,000	$ 2,200,000
Maximum number of shares purchased by employees | shares									200,000
Employee Stock Purchase Plan [Member]
Significant Accounting Policies [Line Items]
Shares authorized | shares	4,000,000								4,000,000
Percentage of employees contribution	15.00%								15.00%
Purchase price of common stock, percentage									95.00%
Aggregate fair market value of shares purchased									$ 25,000
Maximum number of shares purchased by employees | shares									2,000
Shares available for grant | shares	7,900,000								7,900,000
Expiration Lists [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									3 years
Expiration Lists [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									15 years
Non-Compete Agreements [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									3 years
Non-Compete Agreements [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									5 years
Trade Names [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									5 years
Trade Names [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles assets, years									15 years
Claims Handling Operation [Member] | Risk Management [Member]
Significant Accounting Policies [Line Items]
Total revenues									$ 19,600,000	$ 18,000,000	$ 18,000,000