Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Security
Schedule of Investments [Line Items]
Other Invested Assets	$ 2,000,000		$ 2,000,000		$ 1,700,000
Non-credit OTTI on fixed maturities in a net unrealized gain position			400,000		$ 500,000
OTTI losses	0	$ 0	0	$ 1,100,000
Change to cumulative amounts of credit loss portion of OTTI losses	$ 0	0	$ 0	0
Fixed Maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	753		753		454
Gross Unrealized Loss	$ 48,855,000		$ 48,855,000		$ 14,512,000
Cumulative amounts related to credit loss portion of OTTI losses	$ 2,400,000	$ 2,400,000	$ 2,400,000	$ 2,400,000
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security					22
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%		5.00%
Gross Unrealized Loss	$ 1,700,000		$ 1,700,000		$ 700,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities			5 years 3 months 18 days
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	20,494,000		$ 20,494,000		7,338,000
Other Invested Assets
Schedule of Investments [Line Items]
Overseas deposits, fair value	$ 36,600,000		$ 36,600,000		$ 28,800,000