Summary of Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
U.S. Treasury and Agency Obligations
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		$ (56)
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		(167)
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		(407)
Commercial Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		(313)
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		(119)
Foreign Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Total, Gross Unrealized Losses		(12)
Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		85,495	$ 542,881
Less than 12 months, Gross Unrealized Losses		(458)	(13,271)
12 months or longer, Fair Value	[1]	92,782	496,811
12 months or longer, Gross Unrealized Losses	[1]	(635)	(11,252)
Total, Fair Value		178,277	1,039,692
Total, Gross Unrealized Losses		(1,093)	(24,523)
Fixed Maturities | U.S. Treasury and Agency Obligations
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		4,108	0
Less than 12 months, Gross Unrealized Losses		(23)	0
12 months or longer, Fair Value	[1]	13,006	67,185
12 months or longer, Gross Unrealized Losses	[1]	(33)	(1,163)
Total, Fair Value		17,114	67,185
Total, Gross Unrealized Losses		(56)	(1,163)
Fixed Maturities | Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		8,947	22,802
Less than 12 months, Gross Unrealized Losses		(19)	(57)
12 months or longer, Fair Value	[1]	0	28,179
12 months or longer, Gross Unrealized Losses	[1]	0	(281)
Total, Fair Value		8,947	50,981
Total, Gross Unrealized Losses		(19)	(338)
Fixed Maturities | Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		16,537	36,858
Less than 12 months, Gross Unrealized Losses		(141)	(408)
12 months or longer, Fair Value	[1]	3,331	60,838
12 months or longer, Gross Unrealized Losses	[1]	(26)	(1,378)
Total, Fair Value		19,868	97,696
Total, Gross Unrealized Losses		(167)	(1,786)
Fixed Maturities | Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		27,336	96,085
Less than 12 months, Gross Unrealized Losses		(110)	(1,342)
12 months or longer, Fair Value	[1]	34,135	50,506
12 months or longer, Gross Unrealized Losses	[1]	(297)	(670)
Total, Fair Value		61,471	146,591
Total, Gross Unrealized Losses		(407)	(2,012)
Fixed Maturities | Commercial Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		23,225	44,596
Less than 12 months, Gross Unrealized Losses		(114)	(878)
12 months or longer, Fair Value	[1]	25,944	127,557
12 months or longer, Gross Unrealized Losses	[1]	(199)	(2,974)
Total, Fair Value		49,169	172,153
Total, Gross Unrealized Losses		(313)	(3,852)
Fixed Maturities | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		5,139	285,997
Less than 12 months, Gross Unrealized Losses		(50)	(8,791)
12 months or longer, Fair Value	[1]	8,616	115,052
12 months or longer, Gross Unrealized Losses	[1]	(69)	(3,289)
Total, Fair Value		13,755	401,049
Total, Gross Unrealized Losses		(119)	(12,080)
Fixed Maturities | Foreign Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value		203	56,543
Less than 12 months, Gross Unrealized Losses		(1)	(1,795)
12 months or longer, Fair Value	[1]	7,750	47,494
12 months or longer, Gross Unrealized Losses	[1]	(11)	(1,497)
Total, Fair Value		7,953	104,037
Total, Gross Unrealized Losses		$ (12)	$ (3,292)

[1] Fixed maturities in a gross unrealized loss position for twelve months or longer are primarily comprised of non-credit losses on investment grade securities where management does not intend to sell, and it is more likely than not that the Company will not be forced to sell the security before recovery. The Company has analyzed these securities and has determined that they are not other than temporarily impaired.