Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Security
Schedule of Investments [Line Items]
Other Invested Assets	$ 2,000,000		$ 1,700,000
Non-credit OTTI on fixed maturities in a net unrealized gain position	400,000		500,000
Gross Unrealized Loss			$ 16,184,000
OTTI losses	0	$ 1,093,000
Change to cumulative amounts of credit loss portion of OTTI losses	$ 0	0
Fixed Maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	699		454
Gross Unrealized Loss	$ 40,139,000		$ 14,512,000
Cumulative amounts related to credit loss portion of OTTI losses	$ 2,400,000	2,400,000
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security			22
Gross Unrealized Loss			$ 1,672,000
OTTI losses		$ 1,100,000
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%
Gross Unrealized Loss	$ 1,100,000		$ 700,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities	5 years
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 17,484,000		7,338,000
Other Invested Assets
Schedule of Investments [Line Items]
Overseas deposits, fair value	$ 33,600,000		$ 28,800,000