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Credit and Other Debt Agreements - Credit Agreement - Additional Information (Detail) $ in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2022 AUD ($) | Dec. 31, 2022 NZD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2022 AUD ($) | Dec. 31, 2022 NZD ($) | Dec. 31, 2021 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |
Liabilities |  |  | $ 29,717.6 |  |  | $ 24,784.9
Long-term debt |  |  | 16.8 |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, amount outstanding |  |  | 241.9 |  |  |
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 |  |  | 241.9 |  |  |
Revolving Credit Facility [Member] | Before Amendment [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | $ 340,000,000.0 |  |
Revolving Credit Facility [Member] | After Amendment [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  | 410,000,000.0 |  |
Credit Facility Two [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, amount outstanding |  |  |  | 325,000,000.0 | $ 0.0 |
Line of credit facility, remaining borrowing capacity |  |  |  | 25,000,000.0 | 25.0 |
Net borrowings on premium financing debt facility | $ 350,000,000.0 | $ 25.0 |  |  |  |
Credit Facility Three [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, amount outstanding |  |  |  | 22,500,000 |  |
Net borrowings on premium financing debt facility | $ 60,000,000.0 | $ 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 |  |  |  |  | 14.5 |
Line of credit facility, remaining borrowing capacity |  |  |  | $ 37,500,000 | $ 0.5 |
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 |  |  | 4,942.5 |  |  |
Long-term debt |  |  | 5,848.0 |  |  |
Unsecured Multicurrency Credit Agreement [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Estimated fair value of outstanding borrowings |  |  | 60.0 |  |  |
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 | 0.825% | 0.825% |  |  |  |
Annual facility fee of revolving credit facility | 0.125% | 0.125% |  |  |  |
Unsecured Multicurrency Credit Agreement [Member] | Maximum [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rates on base rate loans | 0.40% | 0.40% |  |  |  |
Fixed rate over SOFR rate, eurocurrency rate or RFR rate | 1.40% | 1.40% |  |  |  |
Annual facility fee of revolving credit facility | 0.25% | 0.25% |  |  |  |
Unsecured Multicurrency Credit Agreement [Member] | Standby Letters of Credit [Member] |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | 1,150.6 |  |  |
Line of credit facility, amount outstanding |  |  | 10.6 |  |  |
Liabilities |  |  | 17.4 |  |  |
Line of credit facility, fair value of amount outstanding |  |  | $ 60.0 |  |  |

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