Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 5,249	$ 736
Fair Values, Less than 12 months	199,462	74,488
Losses, More than 12 months	190	118
Fair Values, More than 12 months	62,852	59,538
Total Losses	5,439	854
Total Fair Values	262,314	134,026
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	899	7
Fair Values, Less than 12 months	27,431	697
Total Losses	899	7
Total Fair Values	27,431	697
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	2,927	486
Fair Values, Less than 12 months	78,759	30,538
Total Losses	2,927	486
Total Fair Values	78,759	30,538
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	65
Fair Values, Less than 12 months	4,048
Total Losses	65
Total Fair Values	4,048
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1,358	168
Fair Values, Less than 12 months	89,224	41,056
Losses, More than 12 months	190	118
Fair Values, More than 12 months	62,852	59,538
Total Losses	1,548	286
Total Fair Values	152,076	100,594
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months		75
Fair Values, Less than 12 months		2,197
Total Losses		75
Total Fair Values		$ 2,197