Investment in Unconsolidated Affiliate - Schedule of Investments in Unconsolidated Affiliate (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments In And Advances To Affiliates Schedule Of Investments [Abstract]
Beginning balance			$ 75,716
Equity earnings	$ 2,431	$ 2,512	7,243	$ 7,439
Contributions			6	0
Distributions			(6,090)
Amortization of premium			(231)
Other comprehensive (loss) income	475	$ 831	1,156	$ (84)
Ending balance	$ 77,800		$ 77,800

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.

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

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Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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No definition available.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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