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	6 Months Ended	12 Months Ended
	Jun. 30, 2013 Investment	Dec. 31, 2012 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	367	99
Fair Value	$ 812,704	$ 156,071
Gross Unrealized Loss	29,261	1,261
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Number of Securities	60	12
Fair Value	278,158	42,530
Gross Unrealized Loss	7,492	36
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	60	8
Fair Value	278,158	23,760
Gross Unrealized Loss	7,492	22
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		3
Fair Value		14,118
Gross Unrealized Loss		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	120	14
Fair Value	242,324	24,207
Gross Unrealized Loss	13,114	380
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	110	10
Fair Value	223,186	21,299
Gross Unrealized Loss	11,556	325
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7
Fair Value	17,168
Gross Unrealized Loss	1,546
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	4
Fair Value	1,970	2,908
Gross Unrealized Loss	12	55
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	100	12
Fair Value	129,767	64,187
Gross Unrealized Loss	4,645	204
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	89	10
Fair Value	104,138	62,516
Gross Unrealized Loss	3,096	174
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	9	2
Fair Value	24,070	1,671
Gross Unrealized Loss	1,431	30
Fixed maturities | Agency mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	1,559
Gross Unrealized Loss	118
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	29	41
Fair Value	6,093	9,077
Gross Unrealized Loss	221	549
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	12	6
Fair Value	2,953	1,825
Gross Unrealized Loss	47	22
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	17	35
Fair Value	3,140	7,252
Gross Unrealized Loss	174	527
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	5	2
Fair Value	18,539	2,369
Gross Unrealized Loss	113	49
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4
Fair Value	16,905
Gross Unrealized Loss	29
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	1,634	2,369
Gross Unrealized Loss	84	49
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	10	12
Fair Value	16,353	3,484
Gross Unrealized Loss	514	18
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	7
Fair Value	15,143	2,639
Gross Unrealized Loss	488	7
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	651
Gross Unrealized Loss	10
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	5
Fair Value	559	845
Gross Unrealized Loss	16	11
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	43	6
Fair Value	121,470	10,217
Gross Unrealized Loss	3,162	25
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	43	2
Fair Value	121,470	3,528
Gross Unrealized Loss	3,162	6
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	6	14
Fair Value	33,817	25,288
Gross Unrealized Loss	1,910	626
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	13
Fair Value	33,817	23,345
Gross Unrealized Loss	1,910	522
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		1
Fair Value		1,943
Gross Unrealized Loss		$ 104