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Credit and Other Debt Agreements - Summary of Corporate and Other Debt (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Fixed Rate of 6.44%, Balloon Due August 3, 2017 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 6.44% | 6.44%
Periodic payments of interest and principal, expiry date | 2017 | 2017
Fixed Rate of 2.80%, Balloon Due June 24, 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 2.80% | 2.80%
Periodic payments of interest and principal, expiry date | 2018 | 2018
Fixed Rate of 5.85%, $50 Million Due November 30, 2018 and November 30, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 5.85% | 5.85%
Long-Term debt maturities repayments in 2018 | $ 50 | $ 50
Long-Term debt maturities repayments in 2019 | $ 50 | $ 50
Fixed Rate of 3.20%, Balloon Due June 24, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 3.20% | 3.20%
Periodic payments of interest and principal, expiry date | 2019 | 2019
Fixed Rate of 3.48%, Balloon Due June 24, 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 3.48% | 3.48%
Periodic payments of interest and principal, expiry date | 2020 | 2020
Fixed Rate of 3.99%, Balloon Due July 10, 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 3.99% | 3.99%
Periodic payments of interest and principal, expiry date | 2020 | 2020
Fixed Rate of 5.18%, Balloon Due February 10, 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 5.18% | 5.18%
Periodic payments of interest and principal, expiry date | 2021 | 2021
Fixed Rate of 3.69%, Balloon Due June 14, 2022 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 3.69% | 3.69%
Periodic payments of interest and principal, expiry date | 2022 | 2022
Fixed Rate of 5.49%, Balloon Due February 10, 2023 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 5.49% | 5.49%
Periodic payments of interest and principal, expiry date | 2023 | 2023
Fixed Rate of 4.13%, Balloon Due June 24, 2023 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.13% | 4.13%
Periodic payments of interest and principal, expiry date | 2023 | 2023
Floating Rate of 1.65% LIBOR Plus Balloon Due August 2, 2023 [Member] |  |
Debt Instrument [Line Items] |  |
Quarterly payments of interest rate | 1.65% | 1.65%
Periodic payments of interest and principal, expiry date | 2023 | 2023
Quarterly payments of interest, description | 90 day LIBOR plus | 90 day LIBOR plus
Fixed Rate of 4.58%, Balloon Due February 27, 2024 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.58% | 4.58%
Periodic payments of interest and principal, expiry date | 2024 | 2024
Fixed Rate of 4.31%, Balloon Due June 24, 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.31% | 4.31%
Periodic payments of interest and principal, expiry date | 2025 | 2025
Fixed Rate of 4.73%, Balloon Due February 27, 2026 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.73% | 4.73%
Periodic payments of interest and principal, expiry date | 2026 | 2026
Fixed Rate of 4.40%, Balloon Due June 2, 2026 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.40% | 4.40%
Periodic payments of interest and principal, expiry date | 2026 | 2026
Fixed Rate of 4.36%, Balloon Due June 24, 2026 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.36% | 4.36%
Periodic payments of interest and principal, expiry date | 2026 | 2026
Fixed Rate of 4.09%, Balloon Due June 27, 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.09% | 4.09%
Periodic payments of interest and principal, expiry date | 2027 | 2027
Fixed Rate of 4.09%, Balloon Due August 2, 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.09% | 4.09%
Periodic payments of interest and principal, expiry date | 2027 | 2027
Fixed Rate of 4.14%, Balloon Due August 4, 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.14% | 4.14%
Periodic payments of interest and principal, expiry date | 2027 | 2027
Fixed Rate of 3.46%, Balloon Due December 1, 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 3.46% | 3.46%
Periodic payments of interest and principal, expiry date | 2027 | 2027
Fixed Rate of 4.55%, Balloon Due June 2, 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.55% | 4.55%
Periodic payments of interest and principal, expiry date | 2028 | 2028
Fixed Rate of 4.98%, Balloon Due February 27, 2029 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.98% | 4.98%
Periodic payments of interest and principal, expiry date | 2029 | 2029
Fixed Rate of 4.19%, Balloon Due June 27, 2029 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.19% | 4.19%
Periodic payments of interest and principal, expiry date | 2029 | 2029
Fixed Rate of 4.19%, Balloon Due August 2, 2029 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.19% | 4.19%
Periodic payments of interest and principal, expiry date | 2029 | 2029
Fixed Rate of 4.70%, Balloon Due June 2, 2031 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.70% | 4.70%
Periodic payments of interest and principal, expiry date | 2031 | 2031
Fixed Rate of 4.34%, Balloon Due June 27, 2032 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.34% | 4.34%
Periodic payments of interest and principal, expiry date | 2032 | 2032
Fixed Rate of 4.34%, Balloon Due August 2, 2032 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 4.34% | 4.34%
Periodic payments of interest and principal, expiry date | 2032 | 2032
Prime or LIBOR Plus up to 1.45%, Expires April 8, 2021 [Member] | Credit Agreement [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 1.45% | 1.45%
Periodic payments of interest and principal, expiry date | 2021 | 2021
Premium Financing Debt Facility [Member] |  |
Debt Instrument [Line Items] |  |
Periodic payments of interest and principal, expiry date | 2019 | 2019
First Installment [Member] | Fixed Rate of 5.85%, $50 Million Due November 30, 2018 and November 30, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic payments of interest and principal, expiry date | 2018 | 2018
Second Installment [Member] | Fixed Rate of 5.85%, $50 Million Due November 30, 2018 and November 30, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic payments of interest and principal, expiry date | 2019 | 2019
Facility B [Member] | Premium Financing Debt Facility [Member] | Interbank Rates Plus 1.05%, Expires May 18, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 1.05% | 1.05%
Facility C and D [Member] | Premium Financing Debt Facility [Member] | Interbank Rates Plus 0.55%, Expires May 18, 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Periodic Payment of Interest | 0.55% | 0.55%

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