Securities - Roll-Forward of Other-Than-Temporary Impairment Amount Related to Credit Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Marketable Securities [Abstract]
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, beginning of period	$ 2,071	$ 4,054	$ 4,054
Credit losses previously recognized on securities sold during the period	(2,071)	(1,983)	0
Additional credit loss for which other-than-temporary impairment was not previously recognized	0	0	0
Additional credit loss for which other-than-temporary impairment was previously recognized	0	0	0
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, end of period	$ 0	$ 2,071	$ 4,054

No definition available.

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

+ References

+ Details

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.

+ References

+ Details

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- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

+ Details

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- Definition

This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

+ References

+ Details

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