Debt - Selected Information about Debt Issues (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Outstanding Principal (Par Value)	$ 1,153,259
Outstanding Principal (Book Value)	1,135,040	$ 743,733
Outstanding Principal (Fair Value)	1,234,894
Less current maturity of term loan, par value	1,875
Less current maturity of term loan, book value	1,875	0
Less current maturity of term loan, fair value	1,875
Total long-term debt, par value	1,151,384
Long-term debt	1,133,165	743,733
Total long-term debt, fair value	1,233,019	856,291
Total short-term debt, book value	264,475	490,129
Total short-term debt, fair value	264,475
Total short-term debt, par value	264,725
Total debt, par value	1,416,109
Total debt, book value	1,397,640	1,233,862
Total debt, fair value	$ 1,497,494
Senior Notes, Due 6/15/16
Debt Instrument [Line Items]
Debt instrument, maturity date	Jun. 15, 2016
Annual Interest Rate	6.375%
Issue Date	2006-06
Total short-term debt, book value	$ 0	249,753
Total short-term debt, fair value	0
Total short-term debt, par value	0
Commercial Paper
Debt Instrument [Line Items]
Total short-term debt, book value	262,600	240,376
Total short-term debt, fair value	262,600
Total short-term debt, par value	$ 262,850
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)	164,095	163,920
Outstanding Principal (Fair Value)	$ 196,496
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)	291,424	291,002
Outstanding Principal (Fair Value)	$ 338,363
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)	148,189	147,913
Outstanding Principal (Fair Value)	$ 152,777
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)	$ 125,000
Outstanding Principal (Book Value)	120,929	120,898
Outstanding Principal (Fair Value)	$ 124,378
Junior Subordinated Debentures Due 3/15/36
Debt Instrument [Line Items]
Debt instrument, maturity date	Mar. 15, 2036
Annual Interest Rate	4.26344%
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)	290,403	0
Outstanding Principal (Fair Value)	$ 302,880
Term Loan Due 5/17/21
Debt Instrument [Line Items]
Debt instrument, maturity date	May 17, 2021
Annual Interest Rate	1.85561%
Issue Date	2016-06
Outstanding Principal (Par Value)	$ 100,000
Outstanding Principal (Book Value)	100,000	$ 0
Outstanding Principal (Fair Value)	$ 100,000
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

No definition available.

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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

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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.

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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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- 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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