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, 2011 Investment	Dec. 31, 2010 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	183	313
Fair Value	$ 305,121	$ 757,305
Gross Unrealized Loss	10,782	20,762
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	9	36
Fair Value	65,470	163,253
Gross Unrealized Loss	383	4,499
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	36
Fair Value	58,587	163,253
Gross Unrealized Loss	98	4,499
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	6,883
Gross Unrealized Loss	285
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	13	62
Fair Value	8,530	114,612
Gross Unrealized Loss	108	3,805
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	57
Fair Value	5,894	112,291
Gross Unrealized Loss	72	3,749
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	216	1,004
Gross Unrealized Loss	1	20
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	4
Fair Value	2,420	1,317
Gross Unrealized Loss	35	36
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	3	36
Fair Value	5,087	139,226
Gross Unrealized Loss	3	2,434
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	36
Fair Value	5,087	139,226
Gross Unrealized Loss	3	2,434
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	60	55
Fair Value	22,671	19,154
Gross Unrealized Loss	2,848	2,393
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	3
Fair Value	6,672	3,215
Gross Unrealized Loss	184	20
Fixed maturities | Residential Mortgage Backed Securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7
Fair Value	5,250
Gross Unrealized Loss	313
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	47	52
Fair Value	10,749	15,939
Gross Unrealized Loss	2,351	2,373
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	8	8
Fair Value	11,580	28,177
Gross Unrealized Loss	75	292
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	7
Fair Value	4,933	28,175
Gross Unrealized Loss	12	292
Fixed maturities | Asset-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5
Fair Value	6,645
Gross Unrealized Loss	63
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	2	2
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	12	18
Fair Value	13,808	78,703
Gross Unrealized Loss	593	1,794
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	16
Fair Value	5,465	78,212
Gross Unrealized Loss	29	1,755
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3
Fair Value	6,840
Gross Unrealized Loss	550
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	2
Fair Value	1,503	491
Gross Unrealized Loss	14	39
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	78	98
Fair Value	177,975	214,180
Gross Unrealized Loss	6,772	5,545
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	52	98
Fair Value	135,516	214,180
Gross Unrealized Loss	4,539	5,545
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	18
Fair Value	27,561
Gross Unrealized Loss	1,457
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	8
Fair Value	14,898
Gross Unrealized Loss	776
Common stock
Gain (Loss) on Investments [Line Items]
Number of Securities	5	1
Fair Value	4,949	322
Gross Unrealized Loss	958	10
Common stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	1
Fair Value	3,320	322
Gross Unrealized Loss	587	10
Common stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	1,629
Gross Unrealized Loss	$ 371