Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) AFS_security	Sep. 24, 2016 USD ($)	Sep. 30, 2017 USD ($) AFS_security	Sep. 24, 2016 USD ($)	Dec. 31, 2016 USD ($)
Investments, Debt and Equity Securities [Abstract]
Total realized gains on sales of AFS securities	$ 11,179,000	$ 7,012,000	$ 109,815,000	$ 18,896,000
Total realized losses on sales of AFS securities	$ 856,000	$ 941,000	$ 4,934,000	$ 5,938,000
Number of AFS securities issues contributing to total unrealized loss \| AFS_security	282		282
Total, unrealized Losses	$ 19,083,000		$ 19,083,000		$ 66,971,000

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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