Temporarily Impaired Available for Sale Securities by Time Period Impaired (Detail) - USD ($)	Mar. 31, 2018	Dec. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 2,390,416,000	$ 2,355,037,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	34,765,000	18,735,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,178,440,000	1,289,862,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	38,352,000	23,128,000
Debt Securities, Available-for-sale, Unrealized Loss Position	3,568,856,000	3,644,899,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	73,117,000	41,863,000
Nontaxable Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,506,384,000	1,543,151,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17,243,000	13,827,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	131,685,000	136,217,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,852,000	2,593,000
Debt Securities, Available-for-sale, Unrealized Loss Position	1,638,069,000	1,679,368,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	20,095,000	16,420,000
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	884,032,000	811,886,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17,522,000	4,908,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,046,755,000	1,153,645,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	35,500,000	20,535,000
Debt Securities, Available-for-sale, Unrealized Loss Position	1,930,787,000	1,965,531,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ 53,022,000	$ 25,443,000