Temporarily Impaired Available for Sale Securities by Time Period Impaired (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 710,966	$ 593,647
Less Than 12 Months, Unrealized Losses	16,330	18,968
12 Months or Longer, Fair Value	12,395	7,915
12 Months or Longer, Unrealized Losses	2,634	389
Total, Fair Value	723,361	601,562
Total, Unrealized Losses	18,964	19,357
Tax exempt bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	257,992	138,892
Less Than 12 Months, Unrealized Losses	975	536
12 Months or Longer, Fair Value		6,026
12 Months or Longer, Unrealized Losses		14
Total, Fair Value	257,992	144,918
Total, Unrealized Losses	975	550
Taxable bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	175,903	201,538
Less Than 12 Months, Unrealized Losses	492	1,514
Total, Fair Value	175,903	201,538
Total, Unrealized Losses	492	1,514
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	169,224	166,981
Less Than 12 Months, Unrealized Losses	776	3,019
Total, Fair Value	169,224	166,981
Total, Unrealized Losses	776	3,019
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	107,847	86,236
Less Than 12 Months, Unrealized Losses	14,087	13,899
12 Months or Longer, Fair Value	12,395	1,889
12 Months or Longer, Unrealized Losses	2,634	375
Total, Fair Value	120,242	88,125
Total, Unrealized Losses	$ 16,721	$ 14,274