Debt and Equity Securities (Gross Unrealized Losses) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,137,888	$ 1,580,179
Unrealized losses, Less than 12 months	(15,783)	(41,734)
Estimated fair value, 12 months or longer	560,961	177,875
Unrealized losses, 12 months or longer	(16,124)	(8,174)
Estimated fair value, Total	1,698,849	1,758,054
Unrealized losses, Total	(31,907)	(49,908)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	873,552	1,495,067
Unrealized losses, Less than 12 months	(7,145)	(39,016)
Estimated fair value, 12 months or longer	556,892	176,829
Unrealized losses, 12 months or longer	(15,923)	(8,132)
Estimated fair value, Total	1,430,444	1,671,896
Unrealized losses, Total	(23,068)	(47,148)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	14,107	37,492
Unrealized losses, Less than 12 months	(81)	(685)
Estimated fair value, 12 months or longer	16,465
Unrealized losses, 12 months or longer	(237)
Estimated fair value, Total	30,572	37,492
Unrealized losses, Total	(318)	(685)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	74,804	230,180
Unrealized losses, Less than 12 months	(429)	(8,938)
Estimated fair value, 12 months or longer	31,327	27,687
Unrealized losses, 12 months or longer	(468)	(1,353)
Estimated fair value, Total	106,131	257,867
Unrealized losses, Total	(897)	(10,291)
Debt Securities | Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	31,320	56,579
Unrealized losses, Less than 12 months	(132)	(626)
Estimated fair value, 12 months or longer	3,947
Unrealized losses, 12 months or longer	(31)
Estimated fair value, Total	35,267	56,579
Unrealized losses, Total	(163)	(626)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	32,030	203,011
Unrealized losses, Less than 12 months	(164)	(5,375)
Estimated fair value, 12 months or longer	129,625	131
Unrealized losses, 12 months or longer	(3,097)	(26)
Estimated fair value, Total	161,655	203,142
Unrealized losses, Total	(3,261)	(5,401)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	420,991	838,411
Unrealized losses, Less than 12 months	(2,535)	(20,970)
Estimated fair value, 12 months or longer	357,138	124,425
Unrealized losses, 12 months or longer	(10,853)	(4,284)
Estimated fair value, Total	778,129	962,836
Unrealized losses, Total	(13,388)	(25,254)
Debt Securities | Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,041
Unrealized losses, Less than 12 months	(7)
Estimated fair value, 12 months or longer	6,063	12,086
Unrealized losses, 12 months or longer	(1,050)	(1,803)
Estimated fair value, Total	8,104	12,086
Unrealized losses, Total	(1,057)	(1,803)
Debt Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	298,259	129,394
Unrealized losses, Less than 12 months	(3,797)	(2,422)
Estimated fair value, 12 months or longer	12,327	12,500
Unrealized losses, 12 months or longer	(187)	(666)
Estimated fair value, Total	310,586	141,894
Unrealized losses, Total	(3,984)	(3,088)
Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	264,336	85,112
Unrealized losses, Less than 12 months	(8,638)	(2,718)
Estimated fair value, 12 months or longer	4,069	1,046
Unrealized losses, 12 months or longer	(201)	(42)
Estimated fair value, Total	268,405	86,158
Unrealized losses, Total	$ (8,839)	$ (2,760)