Summary of Consolidated Investments-Other Than Investments in Related Parties (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		$ 3,420,163		$ 3,128,563
Gross Unrealized Gains		57,655		59,175
Gross Unrealized Losses		(16,184)		(30,199)
Amortized Cost		3,378,692		3,099,587
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		3,057,054	[1]	2,635,882
Gross Unrealized Gains		44,158		32,071
Gross Unrealized Losses		(14,512)		(24,414)
Amortized Cost		3,027,408		2,628,225
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		393,563	[1]	273,776
Gross Unrealized Gains		2,081		2,192
Gross Unrealized Losses		(2,014)		(5,128)
Amortized Cost		393,496		276,712
Fixed Maturities | States, Municipalities and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		814,632	[1]	547,415
Gross Unrealized Gains		20,136		11,542
Gross Unrealized Losses		(1,423)		(4,036)
Amortized Cost		795,919		539,909
Fixed Maturities | Agency Mortgage-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		407,619	[1]	487,364
Gross Unrealized Gains		2,352		4,016
Gross Unrealized Losses		(5,414)		(6,585)
Amortized Cost		410,681		489,933
Fixed Maturities | Residential Mortgage Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		54,104	[1]	20,530
Gross Unrealized Gains		606		453
Gross Unrealized Losses		(79)		(55)
Amortized Cost		53,577		20,132
Fixed Maturities | Asset-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		328,753	[1]	314,601
Gross Unrealized Gains		2,138		824
Gross Unrealized Losses		(663)		(1,178)
Amortized Cost		327,278		314,955
Fixed Maturities | Commercial Mortgage-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		160,904	[1]	154,139
Gross Unrealized Gains		2,354		2,859
Gross Unrealized Losses		(1,182)		(1,904)
Amortized Cost		159,732		153,184
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		951,380	[1]	976,634
Gross Unrealized Gains		7,450		8,152
Gross Unrealized Losses		(7,338)		(9,722)
Amortized Cost		951,268		978,204
Fixed Maturities | Corporate Exposures
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[2]	897,479	[1]	838,057
Gross Unrealized Gains	[2]	14,491		10,185
Gross Unrealized Losses	[2]	(3,737)		(5,528)
Amortized Cost	[2]	886,725		833,400
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		235,981	[1]	349,142
Gross Unrealized Gains		13,494		27,016
Gross Unrealized Losses		(1,672)		(5,785)
Amortized Cost		224,159		327,911
Equity Securities | Common Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		52,439	[1]	164,087
Gross Unrealized Gains		7,423		24,677
Gross Unrealized Losses		(112)		(964)
Amortized Cost		45,128		140,374
Equity Securities | Preferred Stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		183,542	[1]	185,055
Gross Unrealized Gains		6,071		2,339
Gross Unrealized Losses		(1,560)		(4,821)
Amortized Cost		179,031		187,537
Short-Term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		127,128	[1]	143,539
Gross Unrealized Gains		3		88
Amortized Cost		127,125		$ 143,451
Investment Including Other Invested Assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[1]	3,421,883
Gross Unrealized Gains		57,655
Gross Unrealized Losses		(16,184)
Amortized Cost		3,380,412
Other Invested Assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[1]	1,720
Amortized Cost		$ 1,720

[1] Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.

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