Debt and Equity Securities (Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 4,799,401	$ 4,603,021
Debt securities, Estimated fair value	4,803,484	4,553,363
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	24,966
Amortized cost, Due after one through five years	48,269
Amortized cost, Due after five through ten years	28,318
Amortized cost, Due after ten years	43,843
Debt Securities, Amortized cost	145,396	155,441
Estimated fair value, Due in one year or less	24,937
Estimated fair value, Due after one through five years	48,104
Estimated fair value, Due after five through ten years	28,338
Estimated fair value, Due after ten years	44,074
Debt securities, Estimated fair value	145,453	151,391
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	61,831
Amortized cost, Due after one through five years	314,910
Amortized cost, Due after five through ten years	254,976
Amortized cost, Due after ten years	436,894
Debt Securities, Amortized cost	1,068,611	1,004,659
Estimated fair value, Due in one year or less	61,960
Estimated fair value, Due after one through five years	318,703
Estimated fair value, Due after five through ten years	259,744
Estimated fair value, Due after ten years	431,955
Debt securities, Estimated fair value	1,072,362	984,333
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	7,772
Amortized cost, Due after one through five years	129,408
Amortized cost, Due after five through ten years	9,699
Amortized cost, Due after ten years	15,671
Debt Securities, Amortized cost	162,550	141,887
Estimated fair value, Due in one year or less	7,783
Estimated fair value, Due after one through five years	128,568
Estimated fair value, Due after five through ten years	9,864
Estimated fair value, Due after ten years	15,434
Debt securities, Estimated fair value	161,649	140,048
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	15,430
Amortized cost, Due after one through five years	106,826
Amortized cost, Due after five through ten years	55,085
Amortized cost, Due after ten years	46,205
Debt Securities, Amortized cost	223,546	197,343
Estimated fair value, Due in one year or less	15,415
Estimated fair value, Due after one through five years	105,946
Estimated fair value, Due after five through ten years	54,607
Estimated fair value, Due after ten years	45,750
Debt securities, Estimated fair value	221,718	193,868
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	29,681
Amortized cost, Due after one through five years	299,209
Amortized cost, Due after five through ten years	314,805
Amortized cost, Due after ten years	72,604
Debt Securities, Amortized cost	716,299	675,683
Estimated fair value, Due in one year or less	29,800
Estimated fair value, Due after one through five years	302,345
Estimated fair value, Due after five through ten years	319,329
Estimated fair value, Due after ten years	75,454
Debt securities, Estimated fair value	726,928	678,524
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	14,843
Amortized cost, Due after one through five years	120,804
Amortized cost, Due after five through ten years	93,769
Amortized cost, Due after ten years	15,361
Debt Securities, Amortized cost	244,777	240,526
Estimated fair value, Due in one year or less	14,869
Estimated fair value, Due after one through five years	121,807
Estimated fair value, Due after five through ten years	96,350
Estimated fair value, Due after ten years	16,428
Debt securities, Estimated fair value	249,454	$ 241,526
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	154,523
Amortized cost, Due after one through five years	1,019,426
Amortized cost, Due after five through ten years	756,652
Amortized cost, Due after ten years	630,578
Debt Securities, Amortized cost	2,561,179
Estimated fair value, Due in one year or less	154,764
Estimated fair value, Due after one through five years	1,025,473
Estimated fair value, Due after five through ten years	768,232
Estimated fair value, Due after ten years	629,095
Debt securities, Estimated fair value	2,577,564
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	2,238,222
Estimated fair value	$ 2,225,920