Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014 Investment	Dec. 31, 2013 Investment
Gain (Loss) on Investments [Line Items]
Number of Securities	15	7
Fair Value	$ 19,928	$ 10,925
Gross Unrealized Loss	1,304	550
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	259	405
Fair Value	731,346	934,607
Gross Unrealized Loss	10,347	32,980
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Number of Securities	50	53
Fair Value	205,598	285,730
Gross Unrealized Loss	5,965	8,855
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	19	27
Fair Value	87,915	136,360
Gross Unrealized Loss	1,061	1,096
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		26
Fair Value		149,370
Gross Unrealized Loss		7,759
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	31
Fair Value	117,683
Gross Unrealized Loss	4,904
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	32	138
Fair Value	53,689	257,641
Gross Unrealized Loss	558	13,651
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	13	28
Fair Value	14,242	40,132
Gross Unrealized Loss	41	297
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	104
Fair Value	2,107	205,152
Gross Unrealized Loss	19	12,100
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	17	6
Fair Value	37,340	12,357
Gross Unrealized Loss	498	1,254
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	52	112
Fair Value	77,182	140,102
Gross Unrealized Loss	998	6,016
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	39
Fair Value	14,743	39,458
Gross Unrealized Loss	52	434
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	64
Fair Value	4,138	77,860
Gross Unrealized Loss	28	3,768
Fixed maturities | Agency mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	46	9
Fair Value	58,301	22,784
Gross Unrealized Loss	918	1,814
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	22	25
Fair Value	7,353	3,848
Gross Unrealized Loss	138	161
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	3
Fair Value	4,966	431
Gross Unrealized Loss	43	2
Fixed maturities | Residential Mortgage Backed Securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	7
Fair Value	659	950
Gross Unrealized Loss	7	29
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	14	15
Fair Value	1,728	2,467
Gross Unrealized Loss	88	130
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	25	15
Fair Value	144,805	76,090
Gross Unrealized Loss	964	480
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	19	14
Fair Value	96,123	75,887
Gross Unrealized Loss	354	479
Fixed maturities | Asset-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	1
Fair Value	14,152	203
Gross Unrealized Loss	185	1
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3
Fair Value	34,530
Gross Unrealized Loss	425
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	8	10
Fair Value	21,132	22,584
Gross Unrealized Loss	98	374
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	4
Fair Value	18,665	6,712
Gross Unrealized Loss	65	31
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	1,076	15,098
Gross Unrealized Loss	6	322
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	4
Fair Value	1,391	774
Gross Unrealized Loss	27	21
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	70	52
Fair Value	221,587	148,612
Gross Unrealized Loss	1,626	3,443
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	52	34
Fair Value	179,390	93,591
Gross Unrealized Loss	797	717
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	18
Fair Value	11,071	55,021
Gross Unrealized Loss	74	2,726
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	14
Fair Value	31,126
Gross Unrealized Loss	755
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	8	7
Fair Value	13,277	10,925
Gross Unrealized Loss	1,254	550
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	5
Fair Value	9,152	7,387
Gross Unrealized Loss	761	422
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	3,887	3,538
Gross Unrealized Loss	486	128
Common Stock | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	238
Gross Unrealized Loss	7
Preferred Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	7
Fair Value	6,651
Gross Unrealized Loss	50
Preferred Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7
Fair Value	6,651
Gross Unrealized Loss	$ 50