Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,106,791	$ 2,937,226
Gross Unrealized Gains	121,126	66,693
Gross Unrealized Losses	(10,499)	(29,398)
Amortized Cost	2,996,164	2,899,931
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,632,313	2,414,210
Gross Unrealized Gains	81,967	40,350
Gross Unrealized Losses	(8,275)	(26,385)
Amortized Cost	2,558,621	2,400,245
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	353,579	252,882
Gross Unrealized Gains	6,019	2,273
Gross Unrealized Losses	(3,860)	(9,214)
Amortized Cost	351,420	259,823
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	553,824	576,859
Gross Unrealized Gains	34,183	21,233
Gross Unrealized Losses	(602)	(781)
Amortized Cost	520,243	556,407
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	504,731	379,269
Gross Unrealized Gains	11,203	5,573
Gross Unrealized Losses	(183)	(2,082)
Amortized Cost	493,711	375,778
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	28,258	30,465
Gross Unrealized Gains	470	694
Gross Unrealized Losses	(130)	(82)
Amortized Cost	27,918	29,853
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	262,903	225,012
Gross Unrealized Gains	1,339	85
Gross Unrealized Losses	(836)	(1,624)
Amortized Cost	262,400	226,551
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	172,579	189,713
Gross Unrealized Gains	6,581	3,119
Gross Unrealized Losses	(1,028)	(1,864)
Amortized Cost	167,026	188,458
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	968,471	824,459
Gross Unrealized Gains	19,593	9,471
Gross Unrealized Losses	(2,177)	(5,652)
Amortized Cost	951,055	820,640
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	756,439	760,010
Gross Unrealized Gains	22,172	7,373
Gross Unrealized Losses	(1,636)	(10,738)
Amortized Cost	735,903	763,375
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	329,635	305,271
Gross Unrealized Gains	39,159	26,341
Gross Unrealized Losses	(2,222)	(3,013)
Amortized Cost	292,698	281,943
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	144,843	217,745
Gross Unrealized Gains		2
Gross Unrealized Losses	(2)
Amortized Cost	$ 144,845	$ 217,743