Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Security
Schedule of Investments [Line Items]
Non-credit OTTI on fixed maturities in a net unrealized gain position	$ 300,000		$ 300,000
OTTI losses	0	$ 0
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	511		808
Gross Unrealized Loss	$ 26,469,000		$ 60,549,000
Cumulative amounts related to credit loss portion of OTTI losses	$ 2,300,000	$ 2,400,000
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%
Gross Unrealized Loss	$ 1,800,000		$ 3,700,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities	5 years 1 month 6 days
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 14,772,000		24,504,000
Other Invested Assets
Schedule of Investments [Line Items]
Equity method investments, cost	2,000,000		2,000,000
Equity method investments, carrying Value	1,900,000		1,900,000
Overseas deposits, fair value	$ 50,400,000		$ 46,500,000