Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail) $ in Millions, $ in Millions, $ in Millions		12 Months Ended
	Jun. 22, 2023 USD ($)	Dec. 31, 2023 AUD ($)	Dec. 31, 2023 NZD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 AUD ($)	Dec. 31, 2023 NZD ($)	Nov. 07, 2023 USD ($)	Dec. 31, 2022 USD ($)
Debt Instrument [Line Items]
Total liabilities				$ 40,800.5				$ 29,168.2
Long-term debt				23.6
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, term	5 years
Line of Credit Facility, Current Borrowing Capacity	$ 1,200.0						$ 1,700.0
Line of credit facility, amount outstanding				289.0
Interest rate for facility		The interest rates on Facility B are Interbank rates, which vary by tranche, duration and currency, plus a margin of 1.500% and 1.850% for the AU$ and NZ$ tranches, respectively. The interest rates on Facilities C and D are 30 day Interbank rates, plus a margin of 0.830% and 0.990% for the AU$ and NZ$ tranches, respectively. The annual fee for Facility B is 0.675% and 0.8325% for the undrawn commitments for the AU$ and NZ$ tranches, respectively. The annual fee for Facility C is 0.77% and for Facility D is 0.90% of the total commitments of the facilities	The interest rates on Facility B are Interbank rates, which vary by tranche, duration and currency, plus a margin of 1.500% and 1.850% for the AU$ and NZ$ tranches, respectively. The interest rates on Facilities C and D are 30 day Interbank rates, plus a margin of 0.830% and 0.990% for the AU$ and NZ$ tranches, respectively. The annual fee for Facility B is 0.675% and 0.8325% for the undrawn commitments for the AU$ and NZ$ tranches, respectively. The annual fee for Facility C is 0.77% and for Facility D is 0.90% of the total commitments of the facilities
Debt instrument fair value amount				289.0
Letter of Credit Sub-facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 75.0
Credit Facility Two [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding					$ 365.0	$ 0.0
Line of credit facility, remaining borrowing capacity					25.0	25.0
Net borrowings on premium financing debt facility		$ 390.0	$ 25.0
Credit Facility Three [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding					45.9
Line of credit facility, remaining borrowing capacity					$ 14.1
Net borrowings on premium financing debt facility		$ 60.0	$ 10.0
Annual fee percentage		0.77%	0.77%
Credit Facility Four [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding						13.7
Line of credit facility, remaining borrowing capacity						$ 1.3
Net borrowings on premium financing debt facility			$ 15.0
Annual fee percentage		0.90%	0.90%
AUD Denominated Tranche [Member] | Revolving Credit Facility [Member] | Facility B [Member]
Debt Instrument [Line Items]
Additional margin percentage on interest rate		1.50%	1.50%
Annual fee percentage		0.675%	0.675%
AUD Denominated Tranche [Member] | Revolving Credit Facility [Member] | Facility C and D [Member]
Debt Instrument [Line Items]
Additional margin percentage on interest rate		0.83%	0.83%
NZD Denominated Tranche [Member] | Revolving Credit Facility [Member] | Facility B [Member]
Debt Instrument [Line Items]
Additional margin percentage on interest rate		1.85%	1.85%
Annual fee percentage		0.8325%	0.8325%
NZD Denominated Tranche [Member] | Revolving Credit Facility [Member] | Facility C and D [Member]
Debt Instrument [Line Items]
Additional margin percentage on interest rate		0.99%	0.99%
Note Purchase Agreements [Member] | Level 3 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value of amount outstanding				6,840.2
Long-term debt				7,498.0
Unsecured Multicurrency Credit Agreement [Member]
Debt Instrument [Line Items]
Estimated fair value of outstanding borrowings				245.0
Unsecured Multicurrency Credit Agreement [Member] | Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,443.4
Line of credit facility, amount outstanding				11.6
Total liabilities				13.7
Line of credit facility, fair value of amount outstanding				$ 245.0
Unsecured Multicurrency Credit Agreement [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rates on base rate loans		0.00%	0.00%
Fixed rate over SOFR rate, eurocurrency rate or RFR rate		0.775%	0.775%
Annual facility fee of revolving credit facility		0.10%	0.10%
Unsecured Multicurrency Credit Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rates on base rate loans		0.375%	0.375%
Fixed rate over SOFR rate, eurocurrency rate or RFR rate		1.375%	1.375%
Annual facility fee of revolving credit facility		0.25%	0.25%