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Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | $ 3,537.3 | $ 3,139.1
Less unamortized debt acquisition costs on Note Purchase Agreements | (6.8) | (6.1)
Semi-annual payments of interest, Net | 3,530.5 | 3,133.0
Note Purchase Agreements [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 3,248.0 | 2,798.0
Fixed Rate of 2.80%, Balloon Due June 24, 2018 [Member] | Note Purchase Agreements [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest |  | 50.0
Fixed Rate of 5.85%, $50 Million Due November 30, 2018 and November 30, 2019 [Member] | Note Purchase Agreements [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 100.0 | 100.0
Fixed Rate of 3.20%, Balloon Due June 24, 2019 [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 June 24, 2020 [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 July 10, 2020 [Member] | Note Purchase Agreements [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 50.0 | 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] |  |
Debt Instrument [Line Items] |  |
Quarterly payments of interest | 50.0 | 50.0
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] |  |
Debt Instrument [Line Items] |  |
Quarterly payments of interest | 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 4.73%, 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 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 |
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 4.44% Balloon Due June 13, 2030 [Member] | Note Purchase Agreements [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 125.0 |
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 4.34%, Balloon Due June 27, 2032 [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] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 125.0 |
Fixed Rate of 4.69% Balloon Due June 13, 2038 [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 75.0 |
Prime or LIBOR Plus up to 1.45%, Expires April 8, 2021 [Member] | Multi Currency Credit Agreement [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 130.0 | 190.0
Interbank Rates Plus 1.05% Expires May 18, 2020 [Member] | AUD Denominated Tranche [Member] | Facility B [Member] | Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 115.9 | 116.4
Interbank Rates Plus 1.05% Expires May 18, 2020 [Member] | AUD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 18.1 | 18.5
Interbank Rates Plus 1.05% Expires May 18, 2020 [Member] | NZD Denominated Tranche [Member] | Facility B [Member] | Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 16.6 | 5.7
Interbank Rates Plus 0.55%, Expires May 18 2020 [Member] | Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | 159.3 | 151.1
Interbank Rates Plus 0.55%, Expires May 18 2020 [Member] | NZD Denominated Tranche [Member] | Facility C and D [Member] | Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Semi-annual payments of interest | $ 8.7 | $ 10.5

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