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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
Sep. 30, 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 $ 1,548,810 849,475
Less than 12 months Gross Unrealized Losses (26,206) (4,666)
Greater than 12 months Fair Value 599,494 505,906
Greater than 12 months Gross Unrealized Losses (27,592) (9,846)
Fair Value 2,148,304 1,355,381
Gross Unrealized Losses (53,798) (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 130,366 273,672
Less than 12 months Gross Unrealized Losses (2,559) (1,502)
Greater than 12 months Fair Value 73,266 54,484
Greater than 12 months Gross Unrealized Losses (1,320) (512)
Fair Value 203,632 328,156
Gross Unrealized Losses (3,879) (2,014)
Fixed Maturities | States, Municipalities and Political Subdivisions    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 267,333 74,097
Less than 12 months Gross Unrealized Losses (4,895) (503)
Greater than 12 months Fair Value 58,882 45,085
Greater than 12 months Gross Unrealized Losses (2,639) (920)
Fair Value 326,215 119,182
Gross Unrealized Losses (7,534) (1,423)
Fixed Maturities | Agency Residential Mortgage-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 72,698 87,496
Less than 12 months Gross Unrealized Losses (2,537) (346)
Greater than 12 months Fair Value 242,433 236,745
Greater than 12 months Gross Unrealized Losses (14,682) (5,068)
Fair Value 315,131 324,241
Gross Unrealized Losses (17,219) (5,414)
Fixed Maturities | Residential Mortgage Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 43,108 12,418
Less than 12 months Gross Unrealized Losses (297) (62)
Greater than 12 months Fair Value 6,501 546
Greater than 12 months Gross Unrealized Losses (196) (17)
Fair Value 49,609 12,964
Gross Unrealized Losses (493) (79)
Fixed Maturities | Asset-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 364,764 85,877
Less than 12 months Gross Unrealized Losses (3,178) (468)
Greater than 12 months Fair Value 38,290 24,733
Greater than 12 months Gross Unrealized Losses (855) (195)
Fair Value 403,054 110,610
Gross Unrealized Losses (4,033) (663)
Fixed Maturities | Commercial Mortgage-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 56,209 20,482
Less than 12 months Gross Unrealized Losses (706) (95)
Greater than 12 months Fair Value 31,868 22,903
Greater than 12 months Gross Unrealized Losses (2,420) (1,087)
Fair Value 88,077 43,385
Gross Unrealized Losses (3,126) (1,182)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 536,779 206,273
Less than 12 months Gross Unrealized Losses (6,718) (971)
Greater than 12 months Fair Value 319,092 284,927
Greater than 12 months Gross Unrealized Losses (18,153) (6,367)
Fair Value 855,871 491,200
Gross Unrealized Losses (24,871) (7,338)
Fixed Maturities | Corporate Exposures    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value [1] 614,332 295,433
Less than 12 months Gross Unrealized Losses [1] (12,034) (1,690)
Greater than 12 months Fair Value [1] 148,254 121,410
Greater than 12 months Gross Unrealized Losses [1] (5,480) (2,047)
Fair Value [1] 762,586 416,843
Gross Unrealized Losses [1] $ (17,514) (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.