Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Total available for sale investments		$ 3,167,095	$ 3,093,156
Gross Unrealized Gains		42,655	18,059
Gross Unrealized Losses		(26,470)	(60,550)
Amortized Cost		3,150,910	3,135,647
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		3,159,808	3,082,923
Gross Unrealized Gains		42,655	18,059
Gross Unrealized Losses		(26,469)	(60,549)
Amortized Cost		3,143,622	3,125,413
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Total available for sale investments		226,093	239,776
Gross Unrealized Gains		1,334	901
Gross Unrealized Losses		(3,050)	(3,957)
Amortized Cost		227,809	242,832
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total available for sale investments		698,980	646,551
Gross Unrealized Gains		21,932	10,734
Gross Unrealized Losses		(586)	(3,448)
Amortized Cost		677,634	639,265
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		329,582	335,542
Gross Unrealized Gains		1,106	843
Gross Unrealized Losses		(8,771)	(13,118)
Amortized Cost		337,247	347,817
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		154,149	138,373
Gross Unrealized Gains		1,538	604
Gross Unrealized Losses		(295)	(530)
Amortized Cost		152,906	138,299
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		532,027	531,991
Gross Unrealized Gains		1,746	1,018
Gross Unrealized Losses		(4,309)	(8,040)
Amortized Cost		534,590	539,013
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		199,450	188,201
Gross Unrealized Gains		2,089	858
Gross Unrealized Losses		(1,397)	(2,816)
Amortized Cost		198,758	190,159
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		1,215,208	1,194,107
Gross Unrealized Gains		6,479	3,323
Gross Unrealized Losses		(14,772)	(24,504)
Amortized Cost		1,223,501	1,215,288
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total available for sale investments	[1]	1,019,527	1,002,489
Gross Unrealized Gains	[1]	12,910	3,101
Gross Unrealized Losses	[1]	(8,061)	(28,640)
Amortized Cost	[1]	1,014,678	1,028,028
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total available for sale investments		7,287	10,233
Gross Unrealized Losses		(1)	(1)
Amortized Cost		$ 7,288	$ 10,234

[1] Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.