Amortized Costs and Fair Values (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized costs	$ 10,870	$ 10,870
Total, Amortized costs	495,146	481,083
Debt securities, Fair values	490,010
Equity securities, Fair values	12,580	11,073
Available for sale securities, Total	502,590	503,259
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	38,945	18,012
Debt securities, Fair values	38,296	19,011
Corporate and utilities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	269,574	268,874
Debt securities, Fair values	276,174	287,528
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	161,417	168,084
Debt securities, Fair values	160,167	169,009
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	14,340	15,243
Debt securities, Fair values	$ 15,373	$ 16,638