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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Fair Value/Carrying Value	$ 65,307	$ 62,493
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	1,500
One to two years	16,694
Two to three years	12,264
Three to five years	5,215
Available-for-sale debt securities, Fair Value/Carrying Value	35,673	32,543
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	490
One to two years	603
Two to three years	1,128
Available-for-sale debt securities, Fair Value/Carrying Value	2,221	2,882
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	4,079
One to two years	2,270
Two to three years	556
Three to five years	1,939
More than ten years	1,475
Available-for-sale debt securities, Fair Value/Carrying Value	10,319	6,526
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Three to five years	1,970
Five to ten years	588
More than ten years	4,649
Available-for-sale debt securities, Fair Value/Carrying Value	7,207	5,509
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	1,519
Two to three years	3,845
Three to five years	3,485
Available-for-sale debt securities, Fair Value/Carrying Value	8,849	14,303
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	729
Two to three years	309
Available-for-sale debt securities, Fair Value/Carrying Value	$ 1,038	$ 730

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