Debt and Equity Securities (Amortized Cost and Estimated Fair Value of Debt Securities) (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,947,204	$ 2,819,817
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,936,709	2,848,594
Debt securities, Estimated fair value	2,947,204	2,819,817
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	10,754
Amortized cost, Due after one through five years	43,020
Amortized cost, Due after five through ten years	26,304
Amortized cost, Due after ten years	2,432
Amortized Cost	82,510
Estimated fair value, Due in one year or less	10,850
Estimated fair value, Due after one through five years	43,199
Estimated fair value, Due after five through ten years	26,491
Estimated fair value, Due after ten years	2,623
Debt securities, Estimated fair value	83,163
U.S. Treasury Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	82,510	66,400
Debt securities, Estimated fair value	83,163	66,384
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	10,130
Amortized cost, Due after one through five years	218,468
Amortized cost, Due after five through ten years	171,734
Amortized cost, Due after ten years	115,869
Amortized Cost	516,201
Estimated fair value, Due in one year or less	10,222
Estimated fair value, Due after one through five years	221,151
Estimated fair value, Due after five through ten years	175,474
Estimated fair value, Due after ten years	116,688
Debt securities, Estimated fair value	523,535
Municipal Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	516,201	491,143
Debt securities, Estimated fair value	523,535	485,965
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	45,372
Amortized cost, Due after one through five years	127,876
Amortized cost, Due after five through ten years	15,984
Amortized cost, Due after ten years	963
Amortized Cost	190,195
Estimated fair value, Due in one year or less	45,639
Estimated fair value, Due after one through five years	129,076
Estimated fair value, Due after five through ten years	15,988
Estimated fair value, Due after ten years	978
Debt securities, Estimated fair value	191,681
Foreign Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	190,195	221,298
Debt securities, Estimated fair value	191,681	222,508
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	6,450
Amortized cost, Due after one through five years	131,967
Amortized cost, Due after five through ten years	78,254
Amortized cost, Due after ten years	8,884
Amortized Cost	225,555
Estimated fair value, Due in one year or less	6,470
Estimated fair value, Due after one through five years	131,174
Estimated fair value, Due after five through ten years	76,848
Estimated fair value, Due after ten years	9,400
Debt securities, Estimated fair value	223,892
Governmental Agency Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	225,555	267,713
Debt securities, Estimated fair value	223,892	262,545
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	22,851
Amortized cost, Due after one through five years	242,589
Amortized cost, Due after five through ten years	150,941
Amortized cost, Due after ten years	27,824
Amortized Cost	444,205
Estimated fair value, Due in one year or less	23,123
Estimated fair value, Due after one through five years	248,490
Estimated fair value, Due after five through ten years	152,788
Estimated fair value, Due after ten years	28,481
Debt securities, Estimated fair value	452,882
Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	444,205	355,893
Debt securities, Estimated fair value	452,882	360,084
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	95,557
Amortized cost, Due after one through five years	763,920
Amortized cost, Due after five through ten years	443,217
Amortized cost, Due after ten years	155,972
Amortized Cost	1,458,666
Estimated fair value, Due in one year or less	96,304
Estimated fair value, Due after one through five years	773,090
Estimated fair value, Due after five through ten years	447,589
Estimated fair value, Due after ten years	158,170
Debt securities, Estimated fair value	1,475,153
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,478,043
Debt securities, Estimated fair value	$ 1,472,051