Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,004,317	$ 2,937,226
Gross Unrealized Gains	90,850	66,693
Gross Unrealized Losses	(17,963)	(29,398)
Amortized Cost	2,931,430	2,899,931
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,477,720	2,414,210
Gross Unrealized Gains	61,200	40,350
Gross Unrealized Losses	(16,317)	(26,385)
Amortized Cost	2,432,837	2,400,245
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	322,009	252,882
Gross Unrealized Gains	4,934	2,273
Gross Unrealized Losses	(4,478)	(9,214)
Amortized Cost	321,553	259,823
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	550,855	576,859
Gross Unrealized Gains	26,085	21,233
Gross Unrealized Losses	(497)	(781)
Amortized Cost	525,267	556,407
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	380,418	379,269
Gross Unrealized Gains	8,831	5,573
Gross Unrealized Losses	(323)	(2,082)
Amortized Cost	371,910	375,778
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	29,451	30,465
Gross Unrealized Gains	569	694
Gross Unrealized Losses	(131)	(82)
Amortized Cost	29,013	29,853
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	235,714	225,012
Gross Unrealized Gains	421	85
Gross Unrealized Losses	(2,040)	(1,624)
Amortized Cost	237,333	226,551
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	188,169	189,713
Gross Unrealized Gains	4,903	3,119
Gross Unrealized Losses	(1,718)	(1,864)
Amortized Cost	184,984	188,458
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	833,752	824,459
Gross Unrealized Gains	14,724	9,471
Gross Unrealized Losses	(4,212)	(5,652)
Amortized Cost	823,240	820,640
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	771,104	760,010
Gross Unrealized Gains	15,457	7,373
Gross Unrealized Losses	(7,130)	(10,738)
Amortized Cost	762,777	763,375
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	314,815	305,271
Gross Unrealized Gains	29,650	26,341
Gross Unrealized Losses	(1,646)	(3,013)
Amortized Cost	286,811	281,943
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	211,782	217,745
Gross Unrealized Gains		2
Amortized Cost	$ 211,782	$ 217,743