Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 3,029.4	$ 2,853.6
Less unamortized debt acquisition costs on Note Purchase Agreements	(6.3)	(5.4)
Semi-annual payments of interest, Net	3,023.1	2,848.2
Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	156.4	125.6
Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	2,798.0	2,450.0
Fixed Rate of 6.44%, Balloon Due August 3, 2017 [Member] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest		300.0
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	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
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
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
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
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
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 4.98%, 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
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
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] \| Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	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
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	75.0	278.0
Interbank Rates Plus 1.05%, Expires May 18, 2019 [Member] \| AUD Denominated Tranche [Member] \| Facility B [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	119.8	100.7
Interbank Rates Plus 1.05%, Expires May 18, 2019 [Member] \| NZD Denominated Tranche [Member] \| Facility B [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	5.9	9.0
Interbank Rates Plus 0.55%, Expires May 18, 2019 [Member] \| AUD Denominated Tranche [Member] \| Facility C and D [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	19.8	5.6
Interbank Rates Plus 0.55%, Expires May 18, 2019 [Member] \| NZD Denominated Tranche [Member] \| Facility C and D [Member] \| Premium Financing Debt Facility [Member]
Debt Instrument [Line Items]
Semi-annual payments of interest	$ 10.9	$ 10.3