Debt and Equity Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 5,928,878	$ 5,761,150
Debt securities, Estimated fair value	5,959,345	5,713,811
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	77,616
Amortized cost, Due after one through five years	59,440
Amortized cost, Due after five through ten years	109
Amortized cost, Due after ten years	11,695
Debt Securities, Amortized cost	148,860	162,904
Estimated fair value, Due in one year or less	77,629
Estimated fair value, Due after one through five years	59,405
Estimated fair value, Due after five through ten years	135
Estimated fair value, Due after ten years	12,645
Debt securities, Estimated fair value	149,814	162,506
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	70,623
Amortized cost, Due after one through five years	172,540
Amortized cost, Due after five through ten years	250,240
Amortized cost, Due after ten years	564,703
Debt Securities, Amortized cost	1,058,106	1,050,134
Estimated fair value, Due in one year or less	70,741
Estimated fair value, Due after one through five years	173,669
Estimated fair value, Due after five through ten years	255,697
Estimated fair value, Due after ten years	578,312
Debt securities, Estimated fair value	1,078,419	1,045,035
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12,427
Amortized cost, Due after one through five years	110,708
Amortized cost, Due after five through ten years	21,071
Amortized cost, Due after ten years	8,333
Debt Securities, Amortized cost	152,539	158,885
Estimated fair value, Due in one year or less	12,419
Estimated fair value, Due after one through five years	111,507
Estimated fair value, Due after five through ten years	21,321
Estimated fair value, Due after ten years	8,105
Debt securities, Estimated fair value	153,352	157,297
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	24,404
Amortized cost, Due after one through five years	123,819
Amortized cost, Due after five through ten years	151,773
Amortized cost, Due after ten years	52,354
Debt Securities, Amortized cost	352,350	319,115
Estimated fair value, Due in one year or less	24,297
Estimated fair value, Due after one through five years	123,194
Estimated fair value, Due after five through ten years	153,607
Estimated fair value, Due after ten years	51,457
Debt securities, Estimated fair value	352,555	316,167
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	40,749
Amortized cost, Due after one through five years	284,110
Amortized cost, Due after five through ten years	246,706
Amortized cost, Due after ten years	25,049
Debt Securities, Amortized cost	596,614	575,646
Estimated fair value, Due in one year or less	40,671
Estimated fair value, Due after one through five years	285,483
Estimated fair value, Due after five through ten years	247,694
Estimated fair value, Due after ten years	25,987
Debt securities, Estimated fair value	599,835	561,260
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	26,900
Amortized cost, Due after one through five years	170,011
Amortized cost, Due after five through ten years	80,814
Amortized cost, Due after ten years	11,013
Debt Securities, Amortized cost	288,738	274,881
Estimated fair value, Due in one year or less	26,880
Estimated fair value, Due after one through five years	170,424
Estimated fair value, Due after five through ten years	81,624
Estimated fair value, Due after ten years	11,160
Debt securities, Estimated fair value	290,088	$ 268,947
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	252,719
Amortized cost, Due after one through five years	920,628
Amortized cost, Due after five through ten years	750,713
Amortized cost, Due after ten years	673,147
Debt Securities, Amortized cost	2,597,207
Estimated fair value, Due in one year or less	252,637
Estimated fair value, Due after one through five years	923,682
Estimated fair value, Due after five through ten years	760,078
Estimated fair value, Due after ten years	687,666
Debt securities, Estimated fair value	2,624,063
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	3,331,671
Estimated fair value	$ 3,335,282