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, 2013 Investment	Dec. 31, 2012 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	326	99
Fair Value	$ 731,167	$ 156,071
Gross Unrealized Loss	25,496	1,261
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Number of Securities	43	12
Fair Value	197,779	42,530
Gross Unrealized Loss	5,694	36
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	42	8
Fair Value	196,851	23,760
Gross Unrealized Loss	5,688	22
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	3
Fair Value	928	14,118
Gross Unrealized Loss	6	11
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		1
Fair Value		4,652
Gross Unrealized Loss		3
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	118	14
Fair Value	235,279	24,207
Gross Unrealized Loss	12,240	380
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	100	10
Fair Value	196,987	21,299
Gross Unrealized Loss	9,485	325
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	17
Fair Value	37,989
Gross Unrealized Loss	2,755
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	4
Fair Value	303	2,908
Gross Unrealized Loss		55
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	82	12
Fair Value	114,012	64,187
Gross Unrealized Loss	3,925	204
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	71	10
Fair Value	90,215	62,516
Gross Unrealized Loss	2,472	174
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	9	2
Fair Value	18,667	1,671
Gross Unrealized Loss	1,354	30
Fixed maturities | Agency mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	5,130
Gross Unrealized Loss	99
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	29	41
Fair Value	4,971	9,077
Gross Unrealized Loss	224	549
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	12	6
Fair Value	2,016	1,825
Gross Unrealized Loss	43	22
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	17	35
Fair Value	2,955	7,252
Gross Unrealized Loss	181	527
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	4	2
Fair Value	32,358	2,369
Gross Unrealized Loss	190	49
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3
Fair Value	32,019
Gross Unrealized Loss	189
Fixed maturities | Asset-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	339
Gross Unrealized Loss	1
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		2
Fair Value		2,369
Gross Unrealized Loss		49
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	11	12
Fair Value	20,780	3,484
Gross Unrealized Loss	365	18
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	7
Fair Value	19,891	2,639
Gross Unrealized Loss	341	7
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	5
Fair Value	889	845
Gross Unrealized Loss	24	11
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	39	6
Fair Value	125,988	10,217
Gross Unrealized Loss	2,858	25
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	37	2
Fair Value	123,673	3,528
Gross Unrealized Loss	2,856	6
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	2,315
Gross Unrealized Loss	2
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		4
Fair Value		6,689
Gross Unrealized Loss		19
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	14	14
Fair Value	35,333	25,288
Gross Unrealized Loss	1,704	626
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	13	13
Fair Value	33,429	23,345
Gross Unrealized Loss	1,340	522
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	1,904	1,943
Gross Unrealized Loss	$ 364	$ 104