Amortized Costs and Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized costs	$ 15,247	$ 10,870
Total, Amortized costs	523,519	481,083
Debt securities, Fair values	515,253
Equity securities, Fair values	17,171	11,073
Available for sale securities, Total	532,424	503,259
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	47,951	18,012
Debt securities, Fair values	47,396	19,011
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	277,619	268,874
Debt securities, Fair values	285,356	287,528
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	168,363	168,084
Debt securities, Fair values	167,174	169,009
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	14,339	15,243
Debt securities, Fair values	$ 15,327	$ 16,638