Gross Unrealized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	$ 24,345	$ 23,030
Gross unrealized losses	926	854
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	627	1,006
Gross unrealized losses	314	7
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	18,781	19,141
Gross unrealized losses	543	486
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	2,374	1,210
Gross unrealized losses	68	286
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	1,279	1,395
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	1,284	278
Gross unrealized losses	$ 1	$ 75