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	Sep. 30, 2014 Investment	Dec. 31, 2013 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	320	405
Fair Value	$ 972,572	$ 934,607
Gross Unrealized Loss	13,528	32,980
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Number of Securities	64	53
Fair Value	263,347	285,730
Gross Unrealized Loss	6,215	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	110,580	136,360
Gross Unrealized Loss	608	1,096
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	9	26
Fair Value	21,694	149,370
Gross Unrealized Loss	727	7,759
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	36
Fair Value	131,073
Gross Unrealized Loss	4,880
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	58	138
Fair Value	120,862	257,641
Gross Unrealized Loss	1,360	13,651
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	18	28
Fair Value	31,874	40,132
Gross Unrealized Loss	189	297
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	104
Fair Value	3,137	205,152
Gross Unrealized Loss	78	12,100
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	39	6
Fair Value	85,851	12,357
Gross Unrealized Loss	1,093	1,254
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	68	112
Fair Value	118,485	140,102
Gross Unrealized Loss	2,363	6,016
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	39
Fair Value	42,373	39,458
Gross Unrealized Loss	230	434
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	64
Fair Value	1,636	77,860
Gross Unrealized Loss	12	3,768
Fixed maturities | Agency mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	61	9
Fair Value	74,476	22,784
Gross Unrealized Loss	2,121	1,814
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	20	25
Fair Value	7,111	3,848
Gross Unrealized Loss	133	161
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	3
Fair Value	5,295	431
Gross Unrealized Loss	43	2
Fixed maturities | Residential Mortgage Backed Securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		7
Fair Value		950
Gross Unrealized Loss		29
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	14	15
Fair Value	1,816	2,467
Gross Unrealized Loss	90	130
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	21	15
Fair Value	135,464	76,090
Gross Unrealized Loss	519	480
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	16	14
Fair Value	93,871	75,887
Gross Unrealized Loss	200	479
Fixed maturities | Asset-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	1
Fair Value	9,930	203
Gross Unrealized Loss	28	1
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	31,663
Gross Unrealized Loss	291
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	13	10
Fair Value	64,423	22,584
Gross Unrealized Loss	588	374
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	9	4
Fair Value	48,884	6,712
Gross Unrealized Loss	408	31
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		2
Fair Value		15,098
Gross Unrealized Loss		322
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	4
Fair Value	15,539	774
Gross Unrealized Loss	180	21
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	76	52
Fair Value	262,880	148,612
Gross Unrealized Loss	2,350	3,443
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	59	34
Fair Value	218,185	93,591
Gross Unrealized Loss	1,016	717
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	18
Fair Value	8,006	55,021
Gross Unrealized Loss	124	2,726
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	11
Fair Value	36,689
Gross Unrealized Loss	1,210
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	7	7
Fair Value	19,650	10,925
Gross Unrealized Loss	772	550
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	5
Fair Value	15,496	7,387
Gross Unrealized Loss	309	422
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	3,940	3,538
Gross Unrealized Loss	433	128
Common Stock | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	214
Gross Unrealized Loss	30
Preferred Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	19
Fair Value	21,383
Gross Unrealized Loss	249
Preferred Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	19
Fair Value	21,383
Gross Unrealized Loss	$ 249