Available for Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,376,091	$ 6,691,140
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	143,533	114,449
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	19,083	66,971
Fair Value	6,500,541	6,738,618
Tax exempt bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,054,341	3,036,060
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	5,308	2,211
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,841	24,649
Fair Value	2,056,808	3,013,622
Taxable bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,185,377	2,469,192
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,639	1,359
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	14,000	33,903
Fair Value	2,175,016	2,436,648
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	164,548	164,548
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	463	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	463
Fair Value	165,011	164,085
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,971,825	1,021,340
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	134,123	110,879
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,242	7,956
Fair Value	$ 2,103,706	$ 1,124,263