Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,233,347	$ 3,128,563
Gross Unrealized Gains	90,193	59,175
Gross Unrealized Losses	(15,865)	(30,199)
Amortized Cost	3,159,019	3,099,587
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,750,404	2,635,882
Gross Unrealized Gains	48,837	32,071
Gross Unrealized Losses	(14,343)	(24,414)
Amortized Cost	2,715,910	2,628,225
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds, and Foreign
Gain (Loss) on Investments [Line Items]
Total investments	264,511	273,776
Gross Unrealized Gains	2,888	2,192
Gross Unrealized Losses	(2,660)	(5,128)
Amortized Cost	264,283	276,712
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	655,064	547,415
Gross Unrealized Gains	19,306	11,542
Gross Unrealized Losses	(2,004)	(4,036)
Amortized Cost	637,762	539,909
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	455,133	487,364
Gross Unrealized Gains	3,849	4,016
Gross Unrealized Losses	(5,389)	(6,585)
Amortized Cost	456,673	489,933
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	18,037	20,530
Gross Unrealized Gains	506	453
Gross Unrealized Losses	(25)	(55)
Amortized Cost	17,556	20,132
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	315,945	314,601
Gross Unrealized Gains	1,915	824
Gross Unrealized Losses	(478)	(1,178)
Amortized Cost	314,508	314,955
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	145,573	154,139
Gross Unrealized Gains	2,883	2,859
Gross Unrealized Losses	(1,432)	(1,904)
Amortized Cost	144,122	153,184
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	934,688	976,634
Gross Unrealized Gains	9,153	8,152
Gross Unrealized Losses	(7,324)	(9,722)
Amortized Cost	932,859	978,204
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total investments	896,141	838,057
Gross Unrealized Gains	17,490	10,185
Gross Unrealized Losses	(2,355)	(5,528)
Amortized Cost	881,006	833,400
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	352,062	349,142
Gross Unrealized Gains	41,073	27,016
Gross Unrealized Losses	(1,522)	(5,785)
Amortized Cost	312,511	327,911
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Total investments	172,456	164,087
Gross Unrealized Gains	30,985	24,677
Gross Unrealized Losses	(1,023)	(964)
Amortized Cost	142,494	140,374
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Total investments	179,606	185,055
Gross Unrealized Gains	10,088	2,339
Gross Unrealized Losses	(499)	(4,821)
Amortized Cost	170,017	187,537
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total investments	130,881	143,539
Gross Unrealized Gains	283	88
Amortized Cost	$ 130,598	$ 143,451