Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,128,563		$ 2,937,226
Gross Unrealized Gains	59,175		66,693
Gross Unrealized Losses	(30,199)		(29,398)
Amortized Cost	3,099,587		2,899,931
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,635,882	[1]	2,414,210
Gross Unrealized Gains	32,071		40,350
Gross Unrealized Losses	(24,414)		(26,385)
Amortized Cost	2,628,225		2,400,245
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	273,776	[1]	252,882
Gross Unrealized Gains	2,192		2,273
Gross Unrealized Losses	(5,128)		(9,214)
Amortized Cost	276,712		259,823
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	547,415	[1]	576,859
Gross Unrealized Gains	11,542		21,233
Gross Unrealized Losses	(4,036)		(781)
Amortized Cost	539,909		556,407
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	487,364	[1]	379,269
Gross Unrealized Gains	4,016		5,573
Gross Unrealized Losses	(6,585)		(2,082)
Amortized Cost	489,933		375,778
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	20,530	[1]	30,465
Gross Unrealized Gains	453		694
Gross Unrealized Losses	(55)		(82)
Amortized Cost	20,132		29,853
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	314,601	[1]	225,012
Gross Unrealized Gains	824		85
Gross Unrealized Losses	(1,178)		(1,624)
Amortized Cost	314,955		226,551
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	154,139	[1]	189,713
Gross Unrealized Gains	2,859		3,119
Gross Unrealized Losses	(1,904)		(1,864)
Amortized Cost	153,184		188,458
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	976,634	[1]	824,459
Gross Unrealized Gains	8,152		9,471
Gross Unrealized Losses	(9,722)		(5,652)
Amortized Cost	978,204		820,640
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	838,057	[1]	760,010
Gross Unrealized Gains	10,185		7,373
Gross Unrealized Losses	(5,528)		(10,738)
Amortized Cost	833,400		763,375
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	349,142	[1]	305,271
Gross Unrealized Gains	27,016		26,341
Gross Unrealized Losses	(5,785)		(3,013)
Amortized Cost	327,911		281,943
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	143,539	[1]	217,745
Gross Unrealized Gains	88		2
Amortized Cost	$ 143,451		$ 217,743

[1] Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.