Debt and Equity Securities (Investments in Debt Securities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized Cost	$ 4,290,864	$ 3,432,466
Debt securities, Estimated fair value	4,279,347	3,450,252
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	25,982
Amortized cost, Due after one through five years	76,874
Amortized cost, Due after five through ten years	11,875
Amortized cost, Due after ten years	15,521
Debt Securities, Amortized Cost	130,252	64,195
Estimated fair value, Due in one year or less	25,959
Estimated fair value, Due after one through five years	76,462
Estimated fair value, Due after five through ten years	11,765
Estimated fair value, Due after ten years	15,186
Debt securities, Estimated fair value	129,372	64,982
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	47,506
Amortized cost, Due after one through five years	298,071
Amortized cost, Due after five through ten years	208,057
Amortized cost, Due after ten years	138,366
Debt Securities, Amortized Cost	692,000	577,703
Estimated fair value, Due in one year or less	47,580
Estimated fair value, Due after one through five years	300,859
Estimated fair value, Due after five through ten years	213,119
Estimated fair value, Due after ten years	142,237
Debt securities, Estimated fair value	703,795	587,677
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	6,169
Amortized cost, Due after one through five years	110,707
Amortized cost, Due after five through ten years	5,461
Amortized cost, Due after ten years	7,647
Debt Securities, Amortized Cost	129,984	133,365
Estimated fair value, Due in one year or less	6,199
Estimated fair value, Due after one through five years	111,643
Estimated fair value, Due after five through ten years	5,464
Estimated fair value, Due after ten years	6,795
Debt securities, Estimated fair value	130,101	134,938
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	31,778
Amortized cost, Due after one through five years	331,052
Amortized cost, Due after five through ten years	28,156
Amortized cost, Due after ten years	28,883
Debt Securities, Amortized Cost	419,869	198,330
Estimated fair value, Due in one year or less	31,768
Estimated fair value, Due after one through five years	329,700
Estimated fair value, Due after five through ten years	28,443
Estimated fair value, Due after ten years	28,180
Debt securities, Estimated fair value	418,091	197,874
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	16,569
Amortized cost, Due after one through five years	298,860
Amortized cost, Due after five through ten years	264,527
Amortized cost, Due after ten years	62,913
Debt Securities, Amortized Cost	642,869	446,630
Estimated fair value, Due in one year or less	16,711
Estimated fair value, Due after one through five years	297,838
Estimated fair value, Due after five through ten years	259,775
Estimated fair value, Due after ten years	60,359
Debt securities, Estimated fair value	634,683	452,129
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	13,340
Amortized cost, Due after one through five years	92,104
Amortized cost, Due after five through ten years	90,555
Amortized cost, Due after ten years	14,163
Debt Securities, Amortized Cost	210,162	183,528
Estimated fair value, Due in one year or less	13,376
Estimated fair value, Due after one through five years	91,836
Estimated fair value, Due after five through ten years	88,310
Estimated fair value, Due after ten years	14,110
Debt securities, Estimated fair value	207,632	$ 183,952
Debt Securities Excluding Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	141,344
Amortized cost, Due after one through five years	1,207,668
Amortized cost, Due after five through ten years	608,631
Amortized cost, Due after ten years	267,493
Debt Securities, Amortized Cost	2,225,136
Estimated fair value, Due in one year or less	141,593
Estimated fair value, Due after one through five years	1,208,338
Estimated fair value, Due after five through ten years	606,876
Estimated fair value, Due after ten years	266,867
Debt securities, Estimated fair value	2,223,674
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	2,065,728
Estimated fair value	$ 2,055,673