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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
Jun. 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 $ 1,602,449 849,475
Less than 12 months Gross Unrealized Losses (29,328) (4,666)
Greater than 12 months Fair Value 444,345 505,906
Greater than 12 months Gross Unrealized Losses (19,527) (9,846)
Fair Value 2,046,794 1,355,381
Gross Unrealized Losses (48,855) (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 220,029 273,672
Less than 12 months Gross Unrealized Losses (3,420) (1,502)
Greater than 12 months Fair Value 39,864 54,484
Greater than 12 months Gross Unrealized Losses (769) (512)
Fair Value 259,893 328,156
Gross Unrealized Losses (4,189) (2,014)
Fixed Maturities | States, Municipalities and Political Subdivisions    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 213,600 74,097
Less than 12 months Gross Unrealized Losses (3,184) (503)
Greater than 12 months Fair Value 42,654 45,085
Greater than 12 months Gross Unrealized Losses (1,686) (920)
Fair Value 256,254 119,182
Gross Unrealized Losses (4,870) (1,423)
Fixed Maturities | Agency Residential Mortgage-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 115,138 87,496
Less than 12 months Gross Unrealized Losses (3,504) (346)
Greater than 12 months Fair Value 214,279 236,745
Greater than 12 months Gross Unrealized Losses (10,993) (5,068)
Fair Value 329,417 324,241
Gross Unrealized Losses (14,497) (5,414)
Fixed Maturities | Residential Mortgage Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 78,032 12,418
Less than 12 months Gross Unrealized Losses (421) (62)
Greater than 12 months Fair Value 387 546
Greater than 12 months Gross Unrealized Losses (10) (17)
Fair Value 78,419 12,964
Gross Unrealized Losses (431) (79)
Fixed Maturities | Asset-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 273,120 85,877
Less than 12 months Gross Unrealized Losses (2,413) (468)
Greater than 12 months Fair Value 23,583 24,733
Greater than 12 months Gross Unrealized Losses (336) (195)
Fair Value 296,703 110,610
Gross Unrealized Losses (2,749) (663)
Fixed Maturities | Commercial Mortgage-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 59,304 20,482
Less than 12 months Gross Unrealized Losses (954) (95)
Greater than 12 months Fair Value 22,124 22,903
Greater than 12 months Gross Unrealized Losses (1,863) (1,087)
Fair Value 81,428 43,385
Gross Unrealized Losses (2,817) (1,182)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value 525,594 206,273
Less than 12 months Gross Unrealized Losses (7,292) (971)
Greater than 12 months Fair Value 260,373 284,927
Greater than 12 months Gross Unrealized Losses (13,202) (6,367)
Fair Value 785,967 491,200
Gross Unrealized Losses (20,494) (7,338)
Fixed Maturities | Corporate Exposures    
Gain (Loss) on Investments [Line Items]    
Less than 12 months Fair Value [1] 643,226 295,433
Less than 12 months Gross Unrealized Losses [1] (15,432) (1,690)
Greater than 12 months Fair Value [1] 101,454 121,410
Greater than 12 months Gross Unrealized Losses [1] (3,870) (2,047)
Fair Value [1] 744,680 416,843
Gross Unrealized Losses [1] $ (19,302) (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.