Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) Security	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Security
Schedule of Investments [Line Items]
Other invested assets	$ 1,776,000		$ 1,960,000
Non-credit OTTI on fixed maturities in a net unrealized gain position	400,000		400,000
Gross Unrealized Loss	22,337,000		$ 30,199,000
OTTI losses		$ 0
Change to cumulative amounts of credit loss portion of OTTI losses	$ 0	0
Equity Securities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	45		75
Gross Unrealized Loss	$ 1,872,000		$ 5,785,000
Fixed maturities
Schedule of Investments [Line Items]
Number of securities in unrealized loss position | Security	398		413
Gross Unrealized Loss	$ 20,465,000		$ 24,414,000
OTTI losses	1,100,000
Cumulative amounts related to credit loss portion of OTTI losses	2,400,000	$ 2,400,000
Non-government-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 1,100,000		$ 1,000,000
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%
Mortgage-backed and asset-backed securities
Schedule of Investments [Line Items]
Securities, effective maturities	4 years 9 months 18 days
Mortgage-backed and asset-backed securities | Fixed maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 9,700,000		$ 9,722,000