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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Fair Value/Carrying Value	$ 35,047	$ 40,799
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	3,127
One to two years	8,389
Two to three years	4,493
Available-for-sale debt securities, Fair Value/Carrying Value	16,009	20,684
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Two to three years	938
Available-for-sale debt securities, Fair Value/Carrying Value	938	1,313
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	872
One to two years	387
Two to three years	976
Three to five years	3,105
Five to ten years	1,038
More than ten years	797
Available-for-sale debt securities, Fair Value/Carrying Value	7,175	5,221
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	289
Three to five years	827
Five to ten years	303
More than ten years	3,122
Available-for-sale debt securities, Fair Value/Carrying Value	4,541	3,791
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	1,691
Two to three years	1,324
Three to five years	1,212
Available-for-sale debt securities, Fair Value/Carrying Value	4,227	9,206
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	1,196
Two to three years	961
Available-for-sale debt securities, Fair Value/Carrying Value	$ 2,157	$ 584

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