Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 1,394	$ 4,062
Fair Values, Less than 12 months	112,288	162,540
Losses, More than 12 months	1,879	2,192
Fair Values, More than 12 months	52,545	76,145
Total Losses	3,273	6,254
Total Fair Values	164,833	238,685
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	267	770
Fair Values, Less than 12 months	17,441	27,686
Losses, More than 12 months	293	43
Fair Values, More than 12 months	15,678	628
Total Losses	560	813
Total Fair Values	33,119	28,314
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	243	1,682
Fair Values, Less than 12 months	26,204	66,776
Losses, More than 12 months	1,445	1,546
Fair Values, More than 12 months	30,146	21,710
Total Losses	1,688	3,228
Total Fair Values	56,350	88,486
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	732	1,539
Fair Values, Less than 12 months	63,711	63,039
Losses, More than 12 months	141	603
Fair Values, More than 12 months	6,721	53,807
Total Losses	873	2,142
Total Fair Values	70,432	116,846
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	52	60
Fair Values, Less than 12 months	3,782	3,772
Total Losses	52	60
Total Fair Values	3,782	3,772
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	100	11
Fair Values, Less than 12 months	1,150	1,267
Total Losses	100	11
Total Fair Values	$ 1,150	$ 1,267