Debt - Selected Information about Debt Issues (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding Principal (Par Value)	$ 1,151,384
Outstanding Principal (Book Value)	1,136,576	$ 1,135,040
Outstanding Principal (Fair Value)	1,232,767
Less current maturity of term loan, par value	4,375
Less current maturity of term loan, book value	4,375	1,875
Less current maturity of term loan, fair value	4,375
Total long-term debt, par value	1,147,009
Long-term debt	1,132,201	1,133,165
Total long-term debt, fair value	1,228,392	1,233,019
Total short-term debt, par value	328,625
Total short-term debt, book value	328,067	264,475
Total short-term debt, fair value	328,067
Total debt, par value	1,475,634
Total debt, book value	1,460,268	1,397,640
Total debt, fair value	1,556,459
Commercial Paper
Debt Instrument [Line Items]
Total short-term debt, par value	324,250
Total short-term debt, book value	323,692	262,600
Total short-term debt, fair value	$ 323,692
Notes, Due 5/15/23
Debt Instrument [Line Items]
Debt instrument, maturity date	May 15, 2023
Annual Interest Rate	7.875%
Issue Date	1993-05
Outstanding Principal (Par Value)	$ 165,612
Outstanding Principal (Book Value)	164,284	164,095
Outstanding Principal (Fair Value)	$ 195,786
Senior Notes, Due 6/15/19
Debt Instrument [Line Items]
Debt instrument, maturity date	Jun. 15, 2019
Annual Interest Rate	9.25%
Issue Date	2009-06
Outstanding Principal (Par Value)	$ 292,647
Outstanding Principal (Book Value)	291,888	291,424
Outstanding Principal (Fair Value)	$ 320,697
Senior Notes, Due 9/15/22
Debt Instrument [Line Items]
Debt instrument, maturity date	Sep. 15, 2022
Annual Interest Rate	3.80%
Issue Date	2012-09
Outstanding Principal (Par Value)	$ 150,000
Outstanding Principal (Book Value)	148,477	148,189
Outstanding Principal (Fair Value)	$ 155,000
Junior Subordinated Debentures Due 12/15/52
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 15, 2052
Annual Interest Rate	5.875%
Issue Date	2012-09
Outstanding Principal (Par Value)	$ 0
Outstanding Principal (Book Value)	0	120,929
Outstanding Principal (Fair Value)	$ 0
Junior Subordinated Debentures Due 3/15/36
Debt Instrument [Line Items]
Debt instrument, maturity date	Mar. 15, 2036
Annual Interest Rate	4.888%
Outstanding Principal (Par Value)	$ 20,000
Outstanding Principal (Book Value)	20,000	20,000
Outstanding Principal (Fair Value)	$ 20,000
Junior Subordinated Debentures Due 3/15/36 \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread	3.30%
Junior Subordinated Debentures Due 6/15/56
Debt Instrument [Line Items]
Debt instrument, maturity date	Jun. 15, 2056
Annual Interest Rate	6.125%
Issue Date	2016-04
Outstanding Principal (Par Value)	$ 300,000
Outstanding Principal (Book Value)	290,460	290,403
Outstanding Principal (Fair Value)	$ 321,120
Junior Subordinated Debentures Due 11/17/57
Debt Instrument [Line Items]
Annual Interest Rate	5.275%
Outstanding Principal (Par Value)	$ 125,000
Outstanding Principal (Book Value)	123,342	0
Outstanding Principal (Fair Value)	$ 122,039
Term Loan Due 5/17/21
Debt Instrument [Line Items]
Debt instrument, maturity date	May 17, 2021
Annual Interest Rate	2.60%
Issue Date	2016-06
Outstanding Principal (Par Value)	$ 98,125
Outstanding Principal (Book Value)	98,125	$ 100,000
Outstanding Principal (Fair Value)	$ 98,125
Term Loan Due 5/17/21 \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread	1.25%

Debt Instrument, Issuance, Month and Year

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- Definition

Long-term Debt, Face Amount

+ References

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

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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

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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- Definition

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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

+ References

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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.

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No definition available.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- 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.

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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.

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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.

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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.

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