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Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Apr. 25, 2018	Mar. 31, 2018	Dec. 31, 2017
7.75% Subordinated Notes due 2045
Subsequent Event [Line Items]
Subordinated Notes percentage		7.75%	7.75%
Subordinated Notes due date		2045	2045
7.875% Subordinated Notes due 2047
Subsequent Event [Line Items]
Subordinated Notes percentage		7.875%	7.875%
Subordinated Notes due date		2047	2047
Fox Paine and Company
Subsequent Event [Line Items]
Advisory services fees estimated		$ 12,500
Estimated percentage of services provided		50.00%
Fox Paine and Company | Other Liabilities
Subsequent Event [Line Items]
Advisory services fees estimated		$ 6,250
Fox Paine and Company | Corporate Expense and Other Accrued Liabilities
Subsequent Event [Line Items]
Advisory services fees estimated		$ 6,250
Subsequent Events | 7.75% Subordinated Notes due 2045
Subsequent Event [Line Items]
Subordinated Notes percentage	7.75%
Subordinated Notes due date	2045
Subsequent Events | 7.875% Subordinated Notes due 2047
Subsequent Event [Line Items]
Subordinated Notes percentage	7.875%
Subordinated Notes due date	2047

X

- Definition

Debt Instrument Maturity Date Year

+ References

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- Definition

Estimated percentage of investment advisory services provided during period.

+ References

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- 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

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- Definition

Amount of fees earned from providing investment advice and research to customers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(a),(d))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

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