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

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Increase for accrued, but unpaid interest on the debt instrument for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

- 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.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of payment greater than the preceding installment payments to be paid at final maturity date of debt.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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Debt - Additional Information (Detail) - USD ($)			12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Nov. 17, 2017	Apr. 05, 2016	Dec. 31, 2017	May 17, 2016
Debt Instrument [Line Items]
Debt, face amount			$ 1,475,634,000
Term Loan Due 5/17/21
Debt Instrument [Line Items]
Interest rate			2.60%
Junior Subordinated Debentures Due 11/17/57 \| Junior Subordinated Debt
Debt Instrument [Line Items]
Debt, face amount	$ 125,000,000
Interest rate	5.275%
Proceeds from issuance of subordinated long-term debt	$ 123,300,000
Junior Subordinated Debentures Due 11/17/57 \| Senior Notes
Debt Instrument [Line Items]
Debt, face amount	$ 125,000,000
Interest rate	5.875%
Accrued interest	$ 143,000
Junior Subordinated Debentures due 2056 \| Junior Subordinated Debt
Debt Instrument [Line Items]
Debt, face amount		$ 300,000,000
Interest rate		6.125%
Proceeds from issuance of subordinated long-term debt		$ 290,000,000
Senior Notes Due 6/15/2016 \| Senior Notes
Debt Instrument [Line Items]
Debt, face amount		$ 250,000,000
Interest rate		6.375%
Accrued interest		$ 8,000,000
Term Loan Due 5/17/21 \| Unsecured Debt
Debt Instrument [Line Items]
Debt, face amount				$ 100,000,000
Balloon payment				75,000,000
Line of Credit \| $750 Million Credit Facility \| Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 750,000,000
London Interbank Offered Rate (LIBOR) \| Term Loan Due 5/17/21
Debt Instrument [Line Items]
Basis spread			1.25%