Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 859	$ 736
Fair Values, Less than 12 months	65,847	74,488
Losses, More than 12 months	67	118
Fair Values, More than 12 months	59,692	59,538
Total Losses	926	854
Total Fair Values	125,539	134,026
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	295	7
Fair Values, Less than 12 months	23,131	697
Losses, More than 12 months	19
Fair Values, More than 12 months	1,297
Total Losses	314	7
Total Fair Values	24,428	697
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	543	486
Fair Values, Less than 12 months	34,715	30,538
Total Losses	543	486
Total Fair Values	34,715	30,538
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	20	168
Fair Values, Less than 12 months	7,234	41,056
Losses, More than 12 months	48	118
Fair Values, More than 12 months	58,395	59,538
Total Losses	68	286
Total Fair Values	65,629	100,594
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1	75
Fair Values, Less than 12 months	767	2,197
Total Losses	1	75
Total Fair Values	$ 767	$ 2,197