Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Total investments	$ 3,181,337	$ 3,128,563
Gross Unrealized Gains	74,493	59,175
Gross Unrealized Losses	(22,337)	(30,199)
Amortized Cost	3,129,181	3,099,587
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,695,277	2,635,882
Gross Unrealized Gains	37,267	32,071
Gross Unrealized Losses	(20,465)	(24,414)
Amortized Cost	2,678,475	2,628,225
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	270,009	273,776
Gross Unrealized Gains	2,110	2,192
Gross Unrealized Losses	(3,671)	(5,128)
Amortized Cost	271,570	276,712
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	604,912	547,415
Gross Unrealized Gains	13,772	11,542
Gross Unrealized Losses	(3,353)	(4,036)
Amortized Cost	594,493	539,909
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	468,255	487,364
Gross Unrealized Gains	3,858	4,016
Gross Unrealized Losses	(7,051)	(6,585)
Amortized Cost	471,448	489,933
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	16,970	20,530
Gross Unrealized Gains	439	453
Gross Unrealized Losses	(37)	(55)
Amortized Cost	16,568	20,132
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	331,567	314,601
Gross Unrealized Gains	1,470	824
Gross Unrealized Losses	(769)	(1,178)
Amortized Cost	330,866	314,955
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	147,353	154,139
Gross Unrealized Gains	2,670	2,859
Gross Unrealized Losses	(1,843)	(1,904)
Amortized Cost	146,526	153,184
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	964,145	976,634
Gross Unrealized Gains	8,437	8,152
Gross Unrealized Losses	(9,700)	(9,722)
Amortized Cost	965,408	978,204
Fixed maturities | Corporate exposures
Gain (Loss) on Investments [Line Items]
Total investments	856,211	838,057
Gross Unrealized Gains	12,948	10,185
Gross Unrealized Losses	(3,741)	(5,528)
Amortized Cost	847,004	833,400
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	362,214	349,142
Gross Unrealized Gains	37,176	27,016
Gross Unrealized Losses	(1,872)	(5,785)
Amortized Cost	326,910	327,911
Equity Securities | Common stocks
Gain (Loss) on Investments [Line Items]
Total investments	169,905	164,087
Gross Unrealized Gains	29,474	24,677
Gross Unrealized Losses	(652)	(964)
Amortized Cost	141,083	140,374
Equity Securities | Preferred stocks
Gain (Loss) on Investments [Line Items]
Total investments	192,309	185,055
Gross Unrealized Gains	7,702	2,339
Gross Unrealized Losses	(1,220)	(4,821)
Amortized Cost	185,827	187,537
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	123,846	143,539
Gross Unrealized Gains	50	88
Amortized Cost	$ 123,796	$ 143,451