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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Fair Value/Carrying Value	$ 61,063	$ 62,493
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	2,800
One to two years	10,122
Two to three years	9,898
Three to five years	2,742
Available-for-sale debt securities, Fair Value/Carrying Value	25,562	32,543
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	287
Two to three years	1,128
Available-for-sale debt securities, Fair Value/Carrying Value	1,415	2,882
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	3,112
One to two years	657
Two to three years	714
Three to five years	2,244
Five to ten years	445
More than ten years	1,183
Available-for-sale debt securities, Fair Value/Carrying Value	8,355	6,526
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Two to three years	425
Three to five years	1,561
Five to ten years	525
More than ten years	2,852
Available-for-sale debt securities, Fair Value/Carrying Value	5,363	5,509
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	11,716
Two to three years	5,565
Three to five years	2,492
Available-for-sale debt securities, Fair Value/Carrying Value	19,773	14,303
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	285
Two to three years	310
Available-for-sale debt securities, Fair Value/Carrying Value	$ 595	$ 730

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