Schedule of Amortized Cost and Estimated Fair Value of Investments (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		$ 1,203,984	$ 1,257,830
Fixed maturities, Gross Unrealized Gains		31,544	1,848
Fixed maturities, Gross Unrealized losses		(1,093)	(24,523)
Fixed maturities, Estimated Fair Value		1,234,435	1,235,155
Other than temporary impairments recognized in AOCI	[1]	0	0
Equity Securities, Amortized Cost		262,367	124,747
Equity Securities, Gross Unrealized Gains		0	0
Equity Securities, Gross Unrealized losses		0	0
Equity Securities, Estimated Fair Value		262,367	124,747
Other invested assets, Amortized Cost		40,052	50,753
Other invested assets, Gross Unrealized Gains		0	0
Other invested assets, Gross Unrealized losses		0	0
Other invested assets, Estimated Fair Value		40,052	50,753
Total, Amortized Cost		1,506,403	1,433,330
Total, Gross Unrealized Gains		31,544	1,848
Total, Gross Unrealized Losses		(1,093)	(24,523)
Total, Estimated Fair Value		1,536,854	1,410,655
U.S. Treasury and Agency Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		158,914	79,766
Fixed maturities, Gross Unrealized Gains		6,624	252
Fixed maturities, Gross Unrealized losses		(56)	(1,163)
Fixed maturities, Estimated Fair Value		165,482	78,855
Other than temporary impairments recognized in AOCI	[1]	0	0
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		54,905	95,629
Fixed maturities, Gross Unrealized Gains		1,146	322
Fixed maturities, Gross Unrealized losses		(19)	(338)
Fixed maturities, Estimated Fair Value		56,032	95,613
Other than temporary impairments recognized in AOCI	[1]	0	0
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		202,699	119,327
Fixed maturities, Gross Unrealized Gains		3,855	313
Fixed maturities, Gross Unrealized losses		(167)	(1,786)
Fixed maturities, Estimated Fair Value		206,387	117,854
Other than temporary impairments recognized in AOCI	[1]	0	0
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		182,502	185,430
Fixed maturities, Gross Unrealized Gains		1,267	336
Fixed maturities, Gross Unrealized losses		(407)	(2,012)
Fixed maturities, Estimated Fair Value		183,362	183,754
Other than temporary impairments recognized in AOCI	[1]	0	0
Commercial Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		238,637	206,236
Fixed maturities, Gross Unrealized Gains		6,529	338
Fixed maturities, Gross Unrealized losses		(313)	(3,852)
Fixed maturities, Estimated Fair Value		244,853	202,722
Other than temporary impairments recognized in AOCI	[1]	0	0
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		265,244	452,692
Fixed maturities, Gross Unrealized Gains		9,613	243
Fixed maturities, Gross Unrealized losses		(119)	(12,080)
Fixed maturities, Estimated Fair Value		274,738	440,855
Other than temporary impairments recognized in AOCI	[1]	0	0
Foreign Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, Amortized Cost		101,083	118,750
Fixed maturities, Gross Unrealized Gains		2,510	44
Fixed maturities, Gross Unrealized losses		(12)	(3,292)
Fixed maturities, Estimated Fair Value		103,581	115,502
Other than temporary impairments recognized in AOCI	[1]	0	$ 0
Common Stock
Debt Securities, Available-for-sale [Line Items]
Equity Securities, Estimated Fair Value		$ 129,400

[1] Represents the total amount of other than temporary impairment losses relating to factors other than credit losses recognized in accumulated other comprehensive income (“AOCI”).