Debt and Equity Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 5,194,579	$ 4,750,362
Debt securities, Estimated fair value	5,119,843	4,752,684
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	31,771
Amortized cost, Due after one through five years	48,201
Amortized cost, Due after five through ten years	40,899
Amortized cost, Due after ten years	51,395
Debt Securities, Amortized cost	172,266	173,049
Estimated fair value, Due in one year or less	31,647
Estimated fair value, Due after one through five years	47,291
Estimated fair value, Due after five through ten years	40,044
Estimated fair value, Due after ten years	50,738
Debt securities, Estimated fair value	169,720	173,998
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	73,033
Amortized cost, Due after one through five years	295,362
Amortized cost, Due after five through ten years	300,483
Amortized cost, Due after ten years	437,730
Debt Securities, Amortized cost	1,106,608	1,031,146
Estimated fair value, Due in one year or less	73,044
Estimated fair value, Due after one through five years	295,138
Estimated fair value, Due after five through ten years	297,958
Estimated fair value, Due after ten years	428,295
Debt securities, Estimated fair value	1,094,435	1,035,937
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	19,550
Amortized cost, Due after one through five years	101,513
Amortized cost, Due after five through ten years	11,382
Amortized cost, Due after ten years	15,844
Debt Securities, Amortized cost	148,289	170,220
Estimated fair value, Due in one year or less	19,520
Estimated fair value, Due after one through five years	100,893
Estimated fair value, Due after five through ten years	11,081
Estimated fair value, Due after ten years	14,426
Debt securities, Estimated fair value	145,920	169,488
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	31,852
Amortized cost, Due after one through five years	100,514
Amortized cost, Due after five through ten years	76,537
Amortized cost, Due after ten years	53,452
Debt Securities, Amortized cost	262,355	212,731
Estimated fair value, Due in one year or less	31,763
Estimated fair value, Due after one through five years	98,014
Estimated fair value, Due after five through ten years	75,784
Estimated fair value, Due after ten years	51,459
Debt securities, Estimated fair value	257,020	211,470
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	21,982
Amortized cost, Due after one through five years	356,138
Amortized cost, Due after five through ten years	311,501
Amortized cost, Due after ten years	71,608
Debt Securities, Amortized cost	761,229	734,409
Estimated fair value, Due in one year or less	21,951
Estimated fair value, Due after one through five years	351,759
Estimated fair value, Due after five through ten years	305,639
Estimated fair value, Due after ten years	69,575
Debt securities, Estimated fair value	748,924	743,215
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	20,650
Amortized cost, Due after one through five years	152,492
Amortized cost, Due after five through ten years	82,081
Amortized cost, Due after ten years	12,895
Debt Securities, Amortized cost	268,118	256,430
Estimated fair value, Due in one year or less	20,639
Estimated fair value, Due after one through five years	151,135
Estimated fair value, Due after five through ten years	80,145
Estimated fair value, Due after ten years	12,655
Debt securities, Estimated fair value	264,574	$ 259,619
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	198,838
Amortized cost, Due after one through five years	1,054,220
Amortized cost, Due after five through ten years	822,883
Amortized cost, Due after ten years	642,924
Debt Securities, Amortized cost	2,718,865
Estimated fair value, Due in one year or less	198,564
Estimated fair value, Due after one through five years	1,044,230
Estimated fair value, Due after five through ten years	810,651
Estimated fair value, Due after ten years	627,148
Debt securities, Estimated fair value	2,680,593
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	2,475,714
Estimated fair value	$ 2,439,250