Investments - Schedule of Other Long-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Other long-term investments	$ 108,559	$ 53,852
Investment in limited partnerships
Investment [Line Items]
Other long-term investments	66,522	51,509
Commercial mortgage participations
Investment [Line Items]
Other long-term investments	39,489	0
Other
Investment [Line Items]
Other long-term investments	$ 2,548	$ 2,343