Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 4,781	$ 736
Fair Values, Less than 12 months	175,059	74,488
Losses, More than 12 months	385	118
Fair Values, More than 12 months	72,127	59,538
Total Losses	5,166	854
Total Fair Values	247,186	134,026
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	852	7
Fair Values, Less than 12 months	31,494	697
Losses, More than 12 months	37
Fair Values, More than 12 months	636
Total Losses	889	7
Total Fair Values	32,130	697
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	2,542	486
Fair Values, Less than 12 months	61,939	30,538
Total Losses	2,542	486
Total Fair Values	61,939	30,538
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	46
Fair Values, Less than 12 months	3,543
Total Losses	46
Total Fair Values	3,543
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1,229	168
Fair Values, Less than 12 months	75,931	41,056
Losses, More than 12 months	348	118
Fair Values, More than 12 months	71,491	59,538
Total Losses	1,577	286
Total Fair Values	147,422	100,594
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	112	75
Fair Values, Less than 12 months	2,152	2,197
Total Losses	112	75
Total Fair Values	$ 2,152	$ 2,197