Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Losses, Less than 12 months	$ 736	$ 2,001
Fair Values, Less than 12 months	74,488	103,806
Losses, More than 12 months	118	1,029
Fair Values, More than 12 months	59,538	24,830
Total Losses	854	3,030
Total Fair Values	134,026	128,636
Municipal
Schedule of Held-to-maturity Securities [Line Items]
Losses, Less than 12 months	7
Fair Values, Less than 12 months	697
Total Losses	7
Total Fair Values	697
Corporate and utilities
Schedule of Held-to-maturity Securities [Line Items]
Losses, Less than 12 months	486	1,944
Fair Values, Less than 12 months	30,538	42,851
Losses, More than 12 months		1,029
Fair Values, More than 12 months		24,830
Total Losses	486	2,973
Total Fair Values	30,538	67,681
Foreign
Schedule of Held-to-maturity Securities [Line Items]
Losses, Less than 12 months	168	53
Fair Values, Less than 12 months	41,056	59,708
Losses, More than 12 months	118
Fair Values, More than 12 months	59,538
Total Losses	286	53
Total Fair Values	100,594	59,708
Equity securities
Schedule of Held-to-maturity Securities [Line Items]
Losses, Less than 12 months	75	4
Fair Values, Less than 12 months	2,197	1,247
Total Losses	75	4
Total Fair Values	$ 2,197	$ 1,247