Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Total investments		$ 3,421,668	$ 3,128,563
Gross Unrealized Gains		98,815	59,175
Gross Unrealized Losses		(11,590)	(30,199)
Amortized Cost		3,334,443	3,099,587
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total investments		2,798,185	2,635,882
Gross Unrealized Gains		56,213	32,071
Gross Unrealized Losses		(10,130)	(24,414)
Amortized Cost		2,752,102	2,628,225
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Total investments		269,851	273,776
Gross Unrealized Gains		8,169	2,192
Gross Unrealized Losses		(607)	(5,128)
Amortized Cost		262,289	276,712
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total investments		684,792	547,415
Gross Unrealized Gains		20,643	11,542
Gross Unrealized Losses		(1,481)	(4,036)
Amortized Cost		665,630	539,909
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		426,441	487,364
Gross Unrealized Gains		3,365	4,016
Gross Unrealized Losses		(4,298)	(6,585)
Amortized Cost		427,374	489,933
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		34,748	20,530
Gross Unrealized Gains		570	453
Gross Unrealized Losses		(27)	(55)
Amortized Cost		34,205	20,132
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		331,646	314,601
Gross Unrealized Gains		2,503	824
Gross Unrealized Losses		(426)	(1,178)
Amortized Cost		329,569	314,955
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		140,601	154,139
Gross Unrealized Gains		2,742	2,859
Gross Unrealized Losses		(1,308)	(1,904)
Amortized Cost		139,167	153,184
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		933,436	976,634
Gross Unrealized Gains		9,180	8,152
Gross Unrealized Losses		(6,059)	(9,722)
Amortized Cost		930,315	978,204
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total investments	[1]	910,106	838,057
Gross Unrealized Gains	[1]	18,221	10,185
Gross Unrealized Losses	[1]	(1,983)	(5,528)
Amortized Cost	[1]	893,868	833,400
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments		375,876	349,142
Gross Unrealized Gains		42,597	27,016
Gross Unrealized Losses		(1,460)	(5,785)
Amortized Cost		334,739	327,911
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Total investments		180,771	164,087
Gross Unrealized Gains		35,179	24,677
Gross Unrealized Losses		(815)	(964)
Amortized Cost		146,407	140,374
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Total investments		195,105	185,055
Gross Unrealized Gains		7,418	2,339
Gross Unrealized Losses		(645)	(4,821)
Amortized Cost		188,332	187,537
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total investments		247,607	143,539
Gross Unrealized Gains		5	88
Amortized Cost		$ 247,602	$ 143,451

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