Investments - Contractual Maturities of Corporate Bonds, Municipal Fixed-Rate Bonds, Asset-Backed Bonds, Mortgage/Agency-Backed Bonds, and Government Bonds (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale-securities, Fair Value/Carrying Value	$ 215,223	$ 296,062
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	23,732
One to two years	35,429
Two to three years	27,642
Available-for-sale-securities, Fair Value/Carrying Value	86,803	111,261
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	16,034
One to two years	27,312
Two to three years	7,310
Three to five years	1,424
Available-for-sale-securities, Fair Value/Carrying Value	52,080	$ 127,787
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Two to three years	1,065
Three to five years	4,422
Available-for-sale-securities, Fair Value/Carrying Value	5,487
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	775
Two to three years	1,000
Five to ten years	684
More than ten years	5,969
Available-for-sale-securities, Fair Value/Carrying Value	8,428
Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
One to two years	4,198
Two to three years	1,878
Three to five years	8,213
Available-for-sale-securities, Fair Value/Carrying Value	$ 14,289