Debt - Selected Information about Debt Issues (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Par Value	$ 1,377,487
Less current maturity of term loan, par value	9,375
Total long-term debt, par value	1,368,112
Total short-term debt, par value	299,375
Total debt, par value	1,667,487
Book Value	1,358,363	$ 1,364,060
Less current maturity of term loan, book value	9,375	6,875
Total long-term debt, book value	1,348,988	1,357,185
Total short-term debt, book value	298,738	307,848
Total debt, book value	1,647,726	1,665,033
Fair Value	1,482,739
Less current maturity of term loan, fair value	9,375
Total long-term debt, fair value	1,473,364	1,384,455
Total short-term debt, fair value	298,738
Total debt, fair value	1,772,102
Commercial Paper
Debt Instrument [Line Items]
Total short-term debt, par value	290,000
Total short-term debt, book value	289,363	300,973
Total short-term debt, fair value	$ 289,363
Notes, Due 5/15/23
Debt Instrument [Line Items]
Coupon Rate	7.875%
Par Value	$ 165,612
Book Value	164,713	164,490
Fair Value	$ 194,525
Senior Notes Due 2022
Debt Instrument [Line Items]
Coupon Rate	3.80%
Par Value	$ 150,000
Book Value	149,089	148,777
Fair Value	153,922
Senior Notes Due 2022 \| Consolidation eliminations \| Subsidiaries
Debt Instrument [Line Items]
Par Value	150,000
Book Value	$ 150,000
Senior Notes, Due 2028
Debt Instrument [Line Items]
Coupon Rate	4.55%
Par Value	$ 550,000
Book Value	543,735	543,169
Fair Value	$ 612,738
Junior Subordinated Debentures Due 2056
Debt Instrument [Line Items]
Coupon Rate	6.125%
Par Value	$ 300,000
Book Value	290,584	290,520
Fair Value	$ 316,920
Junior Subordinated Debentures Due 2057
Debt Instrument [Line Items]
Coupon Rate	5.275%
Par Value	$ 125,000
Book Value	123,367	123,354
Fair Value	$ 117,759
Term loan
Debt Instrument [Line Items]
Coupon Rate	2.952%
Par Value	$ 86,875
Less current maturity of term loan, par value	9,375
Book Value	86,875	93,750
Less current maturity of term loan, book value	9,375	$ 6,875
Fair Value	86,875
Less current maturity of term loan, fair value	$ 9,375
Term loan \| LIBOR
Debt Instrument [Line Items]
Basis spread	1.25%

Long-term Debt, Face Amount

No definition available.

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- Definition

Long-term Debt, Fair Value, Current Maturities

+ References

No definition available.

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- Definition

Long-term Debt, Fair Value, Excluding Current Maturities

+ References

No definition available.

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- Definition

Long-term debt, Face Amount, Current Maturities

+ References

No definition available.

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- Definition

Long-term debt, Face Amount, Excluding Current Maturities

+ References

No definition available.

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- Definition

Short-term debt, face amount

+ References

No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

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-Name Accounting Standards Codification
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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

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