Fair value of the assets less the liabilities of a derivative or group of derivatives.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Name:	us-gaap_DerivativeFairValueOfDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Additional Information) (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
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%	6.05%
Fair value of the interest rate swap associated with the notes	$ 0.3	$ 0.8
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Interest rate on debt	7.00%	7.00%
Interest rate swaps \| 6.05% Subordinated Notes
Debt Outstanding [Line Items]
Fair value of the interest rate swap associated with the notes	$ 0.3		$ 0.8