Debt and Equity Securities (Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 4,262,095	$ 3,432,466
Debt securities, Estimated fair value	4,274,116	3,450,252
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	10,421
Amortized cost, Due after one through five years	78,411
Amortized cost, Due after five through ten years	11,157
Amortized cost, Due after ten years	12,635
Amortized Cost	112,624	64,195
Estimated fair value, Due in one year or less	10,480
Estimated fair value, Due after one through five years	78,593
Estimated fair value, Due after five through ten years	11,150
Estimated fair value, Due after ten years	12,582
Debt securities, Estimated fair value	112,805	64,982
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	40,995
Amortized cost, Due after one through five years	285,582
Amortized cost, Due after five through ten years	219,524
Amortized cost, Due after ten years	136,127
Amortized Cost	682,228	577,703
Estimated fair value, Due in one year or less	41,151
Estimated fair value, Due after one through five years	288,838
Estimated fair value, Due after five through ten years	224,249
Estimated fair value, Due after ten years	138,750
Debt securities, Estimated fair value	692,988	587,677
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	48,824
Amortized cost, Due after one through five years	116,849
Amortized cost, Due after five through ten years	10,812
Amortized cost, Due after ten years	3,061
Amortized Cost	179,546	194,749
Estimated fair value, Due in one year or less	49,112
Estimated fair value, Due after one through five years	118,252
Estimated fair value, Due after five through ten years	10,959
Estimated fair value, Due after ten years	2,987
Debt securities, Estimated fair value	181,310	196,750
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	11,677
Amortized cost, Due after one through five years	338,426
Amortized cost, Due after five through ten years	43,163
Amortized cost, Due after ten years	26,912
Amortized Cost	420,178	198,330
Estimated fair value, Due in one year or less	11,690
Estimated fair value, Due after one through five years	339,130
Estimated fair value, Due after five through ten years	43,633
Estimated fair value, Due after ten years	26,884
Debt securities, Estimated fair value	421,337	197,874
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	20,341
Amortized cost, Due after one through five years	329,518
Amortized cost, Due after five through ten years	368,094
Amortized cost, Due after ten years	77,632
Amortized Cost	795,585	568,774
Estimated fair value, Due in one year or less	20,547
Estimated fair value, Due after one through five years	331,346
Estimated fair value, Due after five through ten years	361,815
Estimated fair value, Due after ten years	74,551
Debt securities, Estimated fair value	788,259	$ 574,269
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	132,258
Amortized cost, Due after one through five years	1,148,786
Amortized cost, Due after five through ten years	652,750
Amortized cost, Due after ten years	256,367
Amortized Cost	2,190,161
Estimated fair value, Due in one year or less	132,980
Estimated fair value, Due after one through five years	1,156,159
Estimated fair value, Due after five through ten years	651,806
Estimated fair value, Due after ten years	255,754
Debt securities, Estimated fair value	2,196,699
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	2,071,934
Estimated fair value	$ 2,077,417