Temporarily Impaired Available for Sale Securities by Time Period Impaired (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 1,538,245	$ 4,507,220
Less Than 12 Months, Unrealized Losses	8,270	62,157
12 Months or Longer, Fair Value	879,886	96,009
12 Months or Longer, Unrealized Losses	10,813	4,814
Total, Fair Value	2,418,131	4,603,229
Total, Unrealized Losses	19,083	66,971
Tax exempt bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	510,790	2,360,143
Less Than 12 Months, Unrealized Losses	1,699	24,416
12 Months or Longer, Fair Value	232,906	6,099
12 Months or Longer, Unrealized Losses	1,142	233
Total, Fair Value	743,696	2,366,242
Total, Unrealized Losses	2,841	24,649
Taxable bonds
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	964,755	1,921,367
Less Than 12 Months, Unrealized Losses	5,332	33,354
12 Months or Longer, Fair Value	643,454	51,769
12 Months or Longer, Unrealized Losses	8,668	549
Total, Fair Value	1,608,209	1,973,136
Total, Unrealized Losses	14,000	33,903
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	164,085
Less Than 12 Months, Unrealized Losses	0	463
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	0	164,085
Total, Unrealized Losses	0	463
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	62,700	61,625
Less Than 12 Months, Unrealized Losses	1,239	3,924
12 Months or Longer, Fair Value	3,526	38,141
12 Months or Longer, Unrealized Losses	1,003	4,032
Total, Fair Value	66,226	99,766
Total, Unrealized Losses	$ 2,242	$ 7,956