Credit Related Impairment Losses Recognized in Earnings on Other Than Temporarily Impaired Securities (Detail) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Changes in Other-Than-Temporary Impairment Losses	$ (3,700,000)	$ 0	$ (4,900,000)
Noncredit-related Losses Recorded in / (Reclassified from) OCI	0	0	700,000
Net Impairment Losses Recognized in Earnings	$ (3,700,000)	$ 0	$ (4,200,000)

Amount before tax of other than temporary impairment loss (OTTI) on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss and attributable to parent entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25362-111560

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34D
-URI http://asc.fasb.org/extlink&oid=27723805&loc=SL6284304-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration