Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,139,388	$ 2,937,226
Gross Unrealized Gains	115,092	66,693
Gross Unrealized Losses	(8,954)	(29,398)
Amortized Cost	3,033,250	2,899,931
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,660,417	2,414,210
Gross Unrealized Gains	76,554	40,350
Gross Unrealized Losses	(7,187)	(26,385)
Amortized Cost	2,591,050	2,400,245
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	262,030	252,882
Gross Unrealized Gains	3,691	2,273
Gross Unrealized Losses	(3,981)	(9,214)
Amortized Cost	262,320	259,823
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	580,324	576,859
Gross Unrealized Gains	29,288	21,233
Gross Unrealized Losses	(769)	(781)
Amortized Cost	551,805	556,407
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	526,069	379,269
Gross Unrealized Gains	11,778	5,573
Gross Unrealized Losses	(105)	(2,082)
Amortized Cost	514,396	375,778
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	27,055	30,465
Gross Unrealized Gains	455	694
Gross Unrealized Losses	(97)	(82)
Amortized Cost	26,697	29,853
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	288,636	225,012
Gross Unrealized Gains	1,503	85
Gross Unrealized Losses	(458)	(1,624)
Amortized Cost	287,591	226,551
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	165,405	189,713
Gross Unrealized Gains	6,392	3,119
Gross Unrealized Losses	(423)	(1,864)
Amortized Cost	159,436	188,458
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	1,007,165	824,459
Gross Unrealized Gains	20,128	9,471
Gross Unrealized Losses	(1,083)	(5,652)
Amortized Cost	988,120	820,640
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	810,898	760,010
Gross Unrealized Gains	23,447	7,373
Gross Unrealized Losses	(1,354)	(10,738)
Amortized Cost	788,805	763,375
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	342,677	305,271
Gross Unrealized Gains	38,538	26,341
Gross Unrealized Losses	(1,767)	(3,013)
Amortized Cost	305,906	281,943
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	134,273	217,745
Gross Unrealized Gains		2
Amortized Cost	134,273	$ 217,743
Other Invested Assets
Gain (Loss) on Investments [Line Items]
Total investments	2,021
Amortized Cost	$ 2,021