Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Debt securities, amortized cost	$ 9,000,753	$ 6,121,004
Debt securities, Estimated fair value	9,095,806	6,354,822
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	46,695
Amortized cost, Due after one through five years	78,036
Amortized cost, Due after five through ten years	25,518
Amortized cost, Due after ten years	22,006
Debt securities, amortized cost	172,255	80,172
Estimated fair value, Due in one year or less	46,776
Estimated fair value, Due after one through five years	77,850
Estimated fair value, Due after five through ten years	25,673
Estimated fair value, Due after ten years	21,444
Debt securities, Estimated fair value	171,743	80,846
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	26,737
Amortized cost, Due after one through five years	97,493
Amortized cost, Due after five through ten years	751,040
Amortized cost, Due after ten years	688,832
Debt securities, amortized cost	1,564,102	1,168,425
Estimated fair value, Due in one year or less	26,884
Estimated fair value, Due after one through five years	102,020
Estimated fair value, Due after five through ten years	771,806
Estimated fair value, Due after ten years	713,091
Debt securities, Estimated fair value	1,613,801	1,248,808
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	52,799
Amortized cost, Due after one through five years	93,254
Amortized cost, Due after five through ten years	76,214
Amortized cost, Due after ten years	10,228
Debt securities, amortized cost	232,495	194,042
Estimated fair value, Due in one year or less	52,826
Estimated fair value, Due after one through five years	94,105
Estimated fair value, Due after five through ten years	74,215
Estimated fair value, Due after ten years	10,229
Debt securities, Estimated fair value	231,375	199,530
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	19,375
Amortized cost, Due after one through five years	80,313
Amortized cost, Due after five through ten years	37,500
Amortized cost, Due after ten years	70,906
Debt securities, amortized cost	208,094	254,248
Estimated fair value, Due in one year or less	19,596
Estimated fair value, Due after one through five years	81,677
Estimated fair value, Due after five through ten years	37,644
Estimated fair value, Due after ten years	72,590
Debt securities, Estimated fair value	211,507	263,922
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	36,886
Amortized cost, Due after one through five years	479,958
Amortized cost, Due after five through ten years	414,830
Amortized cost, Due after ten years	70,775
Debt securities, amortized cost	1,002,449	637,808
Estimated fair value, Due in one year or less	37,292
Estimated fair value, Due after one through five years	492,943
Estimated fair value, Due after five through ten years	417,154
Estimated fair value, Due after ten years	75,000
Debt securities, Estimated fair value	1,022,389	680,697
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	20,621
Amortized cost, Due after one through five years	266,364
Amortized cost, Due after five through ten years	134,173
Amortized cost, Due after ten years	40,025
Debt securities, amortized cost	461,183	384,572
Estimated fair value, Due in one year or less	20,876
Estimated fair value, Due after one through five years	272,278
Estimated fair value, Due after five through ten years	136,133
Estimated fair value, Due after ten years	42,546
Debt securities, Estimated fair value	471,833	$ 406,401
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	203,113
Amortized cost, Due after one through five years	1,095,418
Amortized cost, Due after five through ten years	1,439,275
Amortized cost, Due after ten years	902,772
Debt securities, amortized cost	3,640,578
Estimated fair value, Due in one year or less	204,250
Estimated fair value, Due after one through five years	1,120,873
Estimated fair value, Due after five through ten years	1,462,625
Estimated fair value, Due after ten years	934,900
Debt securities, Estimated fair value	3,722,648
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	5,360,175
Estimated fair value	$ 5,373,158