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Investments - Contractual Maturities of Corporate Bonds, Municipal Fixed-Rate Bonds, Asset-Backed Bonds, Mortgage/Agency-Backed Bonds, U.S. Government Bonds, Foreign Government Bonds and Variable Rate Demand Notes (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale-securities, Fair Value/Carrying Value	$ 198,337	$ 190,723
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	31,566
One to two years	20,265
Two to three years	19,036
Three to five years	1,406
Available-for-sale-securities, Fair Value/Carrying Value	72,273	66,412
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	8,237
One to two years	1,540
Three to five years	1,182
Available-for-sale-securities, Fair Value/Carrying Value	10,959	11,774
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	1,017
Two to three years	2,078
Three to five years	7,416
Five to ten years	1,173
More than ten years	144
Available-for-sale-securities, Fair Value/Carrying Value	11,828	10,206
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	976
Two to three years	726
Five to ten years	1,896
More than ten years	8,715
Available-for-sale-securities, Fair Value/Carrying Value	12,313	13,004
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	750
One to two years	6,365
Two to three years	8,569
Three to five years	12,363
Available-for-sale-securities, Fair Value/Carrying Value	28,047	29,767
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	1,400
One to two years	1,329
Two to three years	550
Available-for-sale-securities, Fair Value/Carrying Value	3,279	3,730
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Three to five years	1,790
Five to ten years	2,000
More than ten years	6,715
Available-for-sale-securities, Fair Value/Carrying Value	$ 10,505	$ 11,855

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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