Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 4,551.6	$ 4,613.6
Less unamortized debt acquisition costs on Note Purchase Agreements	(7.0)	(6.9)
Semi-annual payments of interest, Net	4,544.6	4,606.7
Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	4,348.0	3,923.0
Fixed Rate of 3.48%, Balloon Due June 24, 2020 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		50.0
Fixed Rate of 3.99%, Balloon Due July 10, 2020 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		50.0
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	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
Floating Rate of 1.65% LIBOR Plus Balloon Due August 2, 2023 [Member] | Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		50.0
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
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
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
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
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
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
Prime or LIBOR Plus up to 1.45%, Expires June 7, 2024 [Member] | Multi Currency Credit Agreement [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		520.0
Interbank Rates Plus 1.400% Expires September 15, 2022 [Member] | AUD Denominated Tranche [Member] | Facility B [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	193.1	142.1
Interbank Rates Plus 0.730% Expires September 15, 2022 [Member] | AUD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		18.8
Interbank Rates Plus 0.940% Expires September 15, 2022 [Member] | NZD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	10.5	9.7
Interbank Rates Expires September 15, 2022 [Member] | Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 203.6	$ 170.6