Fair Value Measurements - Additional Information (Detail) - Level 3 [Member] - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total cumulative unrealized losses related to Level3 assets	$ 300,000
Unrealized losses	$ 0	$ 0	$ 0

This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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