Investments - Additional Information (Detail)	Sep. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Fair value of investments, assets managed on behalf of the trade capital providers \| $	$ 154,800,000	$ 131,900,000
Number of securities in an unrealized loss position, total \| Security	8,410
Number of securities in an unrealized loss position for less than one year \| Security	1,949
Number of securities in an unrealized loss position for a period of one year or greater \| Security	182
Transfers between Level 1 and Level 2 securities \| $	$ 0
Transfers between Level 2 and Level 1 securities \| $	$ 0

Amortized cost and fair value debt securities.

No definition available.

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- Definition

Fair Value of Investments, Assets Managed on Behalf of the Trade Capital Provider

+ References

No definition available.

+ Details

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

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- Definition

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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- Definition

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

+ References

No definition available.

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

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