Temporarily Impaired Available for Sale Securities by Time Period Impaired (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 2,494,896	$ 2,355,037
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	35,829	18,735
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,061,149	1,289,862
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	40,304	23,128
Debt Securities, Available-for-sale, Unrealized Loss Position	3,556,045	3,644,899
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	76,133	41,863
Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,335,920	1,543,151
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	13,364	13,827
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	122,314	136,217
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,534	2,593
Debt Securities, Available-for-sale, Unrealized Loss Position	1,458,234	1,679,368
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	15,898	16,420
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,158,976	811,886
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	22,465	4,908
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	938,835	1,153,645
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	37,770	20,535
Debt Securities, Available-for-sale, Unrealized Loss Position	2,097,811	1,965,531
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ 60,235	$ 25,443