Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value			$ 911,581
Less than 12 months Gross Unrealized Losses			(6,271)
Greater than 12 months Fair Value			508,338
Greater than 12 months Gross Unrealized Losses			(9,913)
Fair Value			1,419,919
Gross Unrealized Losses			(16,184)
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		$ 1,426,602	849,475
Less than 12 months Gross Unrealized Losses		(21,886)	(4,666)
Greater than 12 months Fair Value		479,255	505,906
Greater than 12 months Gross Unrealized Losses		(18,253)	(9,846)
Fair Value		1,905,857	1,355,381
Gross Unrealized Losses		(40,139)	(14,512)
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		273,221	273,672
Less than 12 months Gross Unrealized Losses		(3,294)	(1,502)
Greater than 12 months Fair Value		48,016	54,484
Greater than 12 months Gross Unrealized Losses		(788)	(512)
Fair Value		321,237	328,156
Gross Unrealized Losses		(4,082)	(2,014)
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		216,589	74,097
Less than 12 months Gross Unrealized Losses		(2,854)	(503)
Greater than 12 months Fair Value		42,844	45,085
Greater than 12 months Gross Unrealized Losses		(1,700)	(920)
Fair Value		259,433	119,182
Gross Unrealized Losses		(4,554)	(1,423)
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		113,897	87,496
Less than 12 months Gross Unrealized Losses		(3,147)	(346)
Greater than 12 months Fair Value		223,552	236,745
Greater than 12 months Gross Unrealized Losses		(10,319)	(5,068)
Fair Value		337,449	324,241
Gross Unrealized Losses		(13,466)	(5,414)
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		45,912	12,418
Less than 12 months Gross Unrealized Losses		(252)	(62)
Greater than 12 months Fair Value		405	546
Greater than 12 months Gross Unrealized Losses		(14)	(17)
Fair Value		46,317	12,964
Gross Unrealized Losses		(266)	(79)
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		160,208	85,877
Less than 12 months Gross Unrealized Losses		(1,329)	(468)
Greater than 12 months Fair Value		23,794	24,733
Greater than 12 months Gross Unrealized Losses		(372)	(195)
Fair Value		184,002	110,610
Gross Unrealized Losses		(1,701)	(663)
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		59,809	20,482
Less than 12 months Gross Unrealized Losses		(480)	(95)
Greater than 12 months Fair Value		22,411	22,903
Greater than 12 months Gross Unrealized Losses		(1,571)	(1,087)
Fair Value		82,220	43,385
Gross Unrealized Losses		(2,051)	(1,182)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		379,826	206,273
Less than 12 months Gross Unrealized Losses		(5,208)	(971)
Greater than 12 months Fair Value		270,162	284,927
Greater than 12 months Gross Unrealized Losses		(12,276)	(6,367)
Fair Value		649,988	491,200
Gross Unrealized Losses		(17,484)	(7,338)
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	[1]	556,966	295,433
Less than 12 months Gross Unrealized Losses	[1]	(10,530)	(1,690)
Greater than 12 months Fair Value	[1]	118,233	121,410
Greater than 12 months Gross Unrealized Losses	[1]	(3,489)	(2,047)
Fair Value	[1]	675,199	416,843
Gross Unrealized Losses	[1]	$ (14,019)	(3,737)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value			62,106
Less than 12 months Gross Unrealized Losses			(1,605)
Greater than 12 months Fair Value			2,432
Greater than 12 months Gross Unrealized Losses			(67)
Fair Value			64,538
Gross Unrealized Losses			(1,672)
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value			11,245
Less than 12 months Gross Unrealized Losses			(81)
Greater than 12 months Fair Value			1,770
Greater than 12 months Gross Unrealized Losses			(31)
Fair Value			13,015
Gross Unrealized Losses			(112)
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value			50,861
Less than 12 months Gross Unrealized Losses			(1,524)
Greater than 12 months Fair Value			662
Greater than 12 months Gross Unrealized Losses			(36)
Fair Value			51,523
Gross Unrealized Losses			$ (1,560)

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