Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2021	May 20, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 5,532.1		$ 4,551.6
Less unamortized debt acquisition costs on Senior Notes and Note Purchase Agreements	(15.6)		(7.0)
Less unamortized discount on Bonds Payable	(10.0)
Semi-annual payments of interest, Net	5,506.5		4,544.6
Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	4,448.0		4,348.0
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	850.0	$ 1,500.0
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] \| Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	850.0	$ 850.0
Fixed Rate of 5.18%, Balloon Due February 10, 2021 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest			75.0
Fixed Rate of 3.69%, Balloon Due June 14, 2022 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200.0		200.0
Fixed Rate of 5.49%, Balloon Due February 10, 2023 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50.0		50.0
Fixed Rate of 4.13%, Balloon Due June 24, 2023 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200.0		200.0
Fixed Rate of 4.72%, Balloon due February 13, 2024 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100.0		100.0
Fixed Rate of 4.58%, Balloon Due February 27, 2024 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	325.0		325.0
Floating Interest Rate of 1.40% LIBOR Plus Balloon Due June 13, 2024 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50.0		50.0
Fixed Rate of 4.31%, Balloon Due June 24, 2025 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	200.0		200.0
Fixed Rate of 4.85%, Balloon due February 13, 2026 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	140.0		140.0
Fixed Rate of 4.73%, Balloon Due February 27, 2026 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175.0		175.0
Fixed Rate of 4.40%, Balloon Due June 2, 2026 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175.0		175.0
Fixed Rate of 4.36%, Balloon Due June 24, 2026 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	150.0		150.0
Fixed Rate of 3.75%, Balloon Due January 30, 2027 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	30.0		30.0
Fixed Rate of 4.09%, Balloon Due June 27, 2027 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125.0		125.0
Fixed Rate of 4.09%, Balloon Due August 2, 2027 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125.0		125.0
Fixed Rate of 4.14%, Balloon Due August 4, 2027 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	98.0		98.0
Fixed Rate of 3.46%, Balloon Due December 1, 2027 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100.0		100.0
Fixed Rate of 4.55%, Balloon Due June 2, 2028 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75.0		75.0
Fixed Rate of 4.34%, Balloon due June 13, 2028 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125.0		125.0
Fixed Rate of 5.04%, Balloon due February 13, 2029 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100.0		100.0
Fixed Rate of 4.98%, Balloon Due February 27, 2029 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100.0		100.0
Fixed Rate of 4.19%, Balloon Due June 27, 2029 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50.0		50.0
Fixed Rate of 4.19%, Balloon Due August 2, 2029 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50.0		50.0
Fixed Rate of 3.48%, Balloon Due December 2, 2029 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	50.0		50.0
Fixed Rate of 3.99%, Balloon Due January 30, 2030 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	341.0		341.0
Fixed Rate of 4.44%, Balloon Due June 13, 2030 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125.0		125.0
Fixed Rate of 5.14%, Balloon due March 13, 2031 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	180.0		180.0
Fixed Rate of 4.70%, Balloon Due June 2, 2031 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	25.0		25.0
Fixed Rate of 4.09%, Balloon Due January 30, 2032 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	69.0		69.0
Fixed Rate of 4.34%, Balloon Due June 27, 2032 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75.0		75.0
Fixed Rate of 4.34%, Balloon Due August 2, 2032 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75.0		75.0
Fixed Rate of 4.59%, Balloon due June 13, 2033 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	125.0		125.0
Fixed Rate of 5.29%, balloon due March 13, 2034 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	40.0		40.0
Fixed Rate of 4.48%, Balloon Due June 12, 2034 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	175.0		175.0
Fixed Rate of 4.24%, Balloon Due January 30, 2035 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	79.0		79.0
Fixed Rate of 2.44%, Balloon Due February 10, 2036 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	100.0
Fixed Rate of 2.46%, Balloon Due May 5, 2036 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75.0
Fixed Rate of 4.69%, Balloon Due June 13, 2038 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	75.0		75.0
Fixed Rate of 5.45%, balloon due March 13, 2039 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	40.0		40.0
Interbank Rates Plus 1.200% Expires September 15, 2023 [Member] \| AUD Denominated Tranche [Member] \| Facility B [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	211.9		193.1
Fixed Rate of 4.49%, Balloon Due January 30, 2040 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	56.0		56.0
Interbank Rates Plus 0.680% Expires September 15, 2023 [Member] \| AUD Denominated Tranche [Member] \| Facility C and D [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	11.8
Interbank Rates Plus 0.940% Expires September 15, 2023 [Member] \| NZD Denominated Tranche [Member] \| Facility C and D [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	10.4		10.5
Interbank Rates Expires September 15, 2023 [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 234.1		$ 203.6