Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Additional Information) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Debt Outstanding [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.82%	1.39%
Long-term Debt [Member]
Debt Outstanding [Line Items]
Interest expense on debt	$ 8.0
3.50% Senior Notes
Debt Outstanding [Line Items]
Interest rate on debt	3.50%	3.50%
5.375% Senior Notes
Debt Outstanding [Line Items]
Interest rate on debt	5.375%	5.375%
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Interest rate on debt	6.05%
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Interest rate on debt	7.00%

Debt Outstanding [Line Items]

No definition available.

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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

+ Details

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ 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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Details

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