Debt and Equity Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,109,478	$ 6,121,004
Debt securities, Estimated fair value	7,211,498	6,354,822
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	44,026
Amortized cost, Due after one through five years	73,336
Amortized cost, Due after five through ten years	24,774
Amortized cost, Due after ten years	16,212
Debt Securities, Amortized cost	158,348	80,172
Estimated fair value, Due in one year or less	44,451
Estimated fair value, Due after one through five years	73,186
Estimated fair value, Due after five through ten years	23,789
Estimated fair value, Due after ten years	14,691
Debt securities, Estimated fair value	156,117	80,846
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	37,990
Amortized cost, Due after one through five years	105,276
Amortized cost, Due after five through ten years	540,808
Amortized cost, Due after ten years	722,017
Debt Securities, Amortized cost	1,406,091	1,168,425
Estimated fair value, Due in one year or less	38,272
Estimated fair value, Due after one through five years	110,007
Estimated fair value, Due after five through ten years	562,538
Estimated fair value, Due after ten years	742,801
Debt securities, Estimated fair value	1,453,618	1,248,808
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	39,784
Amortized cost, Due after one through five years	74,345
Amortized cost, Due after five through ten years	77,959
Amortized cost, Due after ten years	14,201
Debt Securities, Amortized cost	206,289	194,042
Estimated fair value, Due in one year or less	39,813
Estimated fair value, Due after one through five years	75,496
Estimated fair value, Due after five through ten years	75,807
Estimated fair value, Due after ten years	14,176
Debt securities, Estimated fair value	205,292	199,530
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	23,877
Amortized cost, Due after one through five years	143,684
Amortized cost, Due after five through ten years	52,930
Amortized cost, Due after ten years	78,856
Debt Securities, Amortized cost	299,347	254,248
Estimated fair value, Due in one year or less	24,068
Estimated fair value, Due after one through five years	146,765
Estimated fair value, Due after five through ten years	53,574
Estimated fair value, Due after ten years	79,098
Debt securities, Estimated fair value	303,505	263,922
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	11,474
Amortized cost, Due after one through five years	329,634
Amortized cost, Due after five through ten years	230,522
Amortized cost, Due after ten years	67,245
Debt Securities, Amortized cost	638,875	637,808
Estimated fair value, Due in one year or less	11,547
Estimated fair value, Due after one through five years	347,060
Estimated fair value, Due after five through ten years	233,607
Estimated fair value, Due after ten years	68,120
Debt securities, Estimated fair value	660,334	680,697
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12,807
Amortized cost, Due after one through five years	223,422
Amortized cost, Due after five through ten years	124,604
Amortized cost, Due after ten years	46,266
Debt Securities, Amortized cost	407,099	384,572
Estimated fair value, Due in one year or less	12,927
Estimated fair value, Due after one through five years	232,283
Estimated fair value, Due after five through ten years	125,873
Estimated fair value, Due after ten years	45,699
Debt securities, Estimated fair value	416,782	$ 406,401
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	169,958
Amortized cost, Due after one through five years	949,697
Amortized cost, Due after five through ten years	1,051,597
Amortized cost, Due after ten years	944,797
Debt Securities, Amortized cost	3,116,049
Estimated fair value, Due in one year or less	171,078
Estimated fair value, Due after one through five years	984,797
Estimated fair value, Due after five through ten years	1,075,188
Estimated fair value, Due after ten years	964,585
Debt securities, Estimated fair value	3,195,648
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	3,993,429
Estimated fair value	$ 4,015,850