Debt and Equity Securities (Amortized Cost and Estimated Fair Value of Debt Securities) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 18,816
Debt securities, Estimated fair value	2,825,477	2,819,817
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,835,448	2,848,594
Debt securities, Estimated fair value	2,825,477	2,819,817
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	13,138
Amortized cost, Due after one through five years	42,970
Amortized cost, Due after five through ten years	6,499
Amortized cost, Due after ten years	3,935
Amortized Cost	66,542
Estimated fair value, Due in one year or less	13,265
Estimated fair value, Due after one through five years	43,014
Estimated fair value, Due after five through ten years	6,455
Estimated fair value, Due after ten years	4,070
Debt securities, Estimated fair value	66,804
U.S. Treasury Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	66,542	66,400
Debt securities, Estimated fair value	66,804	66,384
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12,508
Amortized cost, Due after one through five years	205,407
Amortized cost, Due after five through ten years	164,577
Amortized cost, Due after ten years	122,188
Amortized Cost	504,680
Estimated fair value, Due in one year or less	12,554
Estimated fair value, Due after one through five years	207,254
Estimated fair value, Due after five through ten years	165,724
Estimated fair value, Due after ten years	120,507
Debt securities, Estimated fair value	506,039
Municipal Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	504,680	491,143
Debt securities, Estimated fair value	506,039	485,965
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	30,497
Amortized cost, Due after one through five years	144,461
Amortized cost, Due after five through ten years	12,821
Amortized cost, Due after ten years	1,226
Amortized Cost	189,005
Estimated fair value, Due in one year or less	30,638
Estimated fair value, Due after one through five years	145,736
Estimated fair value, Due after five through ten years	12,729
Estimated fair value, Due after ten years	1,243
Debt securities, Estimated fair value	190,346
Foreign Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	189,005	221,298
Debt securities, Estimated fair value	190,346	222,508
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	10,591
Amortized cost, Due after one through five years	164,673
Amortized cost, Due after five through ten years	78,261
Amortized cost, Due after ten years	8,906
Amortized Cost	262,431
Estimated fair value, Due in one year or less	10,619
Estimated fair value, Due after one through five years	163,292
Estimated fair value, Due after five through ten years	76,121
Estimated fair value, Due after ten years	9,082
Debt securities, Estimated fair value	259,114
Governmental Agency Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	262,431	267,713
Debt securities, Estimated fair value	259,114	262,545
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	24,348
Amortized cost, Due after one through five years	233,512
Amortized cost, Due after five through ten years	73,272
Amortized cost, Due after ten years	5,842
Amortized Cost	336,974
Estimated fair value, Due in one year or less	24,650
Estimated fair value, Due after one through five years	238,333
Estimated fair value, Due after five through ten years	74,220
Estimated fair value, Due after ten years	6,346
Debt securities, Estimated fair value	343,549
Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	336,974	355,893
Debt securities, Estimated fair value	343,549	360,084
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	91,082
Amortized cost, Due after one through five years	791,023
Amortized cost, Due after five through ten years	335,430
Amortized cost, Due after ten years	142,097
Amortized Cost	1,359,632
Estimated fair value, Due in one year or less	91,726
Estimated fair value, Due after one through five years	797,629
Estimated fair value, Due after five through ten years	335,249
Estimated fair value, Due after ten years	141,248
Debt securities, Estimated fair value	1,365,852
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,475,816
Debt securities, Estimated fair value	$ 1,459,625