Debt and Equity Securities (Gross Unrealized Losses) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,551,981	$ 418,285
Unrealized losses, Less than 12 months	(39,185)	(4,169)
Estimated fair value, 12 months or longer	110,755	39,990
Unrealized losses, 12 months or longer	(6,430)	(3,721)
Estimated fair value, Total	1,662,736	458,275
Unrealized losses, Total	(45,615)	(7,890)
Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	99,080	34,258
Unrealized losses, Less than 12 months	(4,050)	(447)
Estimated fair value, 12 months or longer	1,287	98
Unrealized losses, 12 months or longer	(276)	(2)
Estimated fair value, Total	100,367	34,356
Unrealized losses, Total	(4,326)	(449)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,452,901	384,027
Unrealized losses, Less than 12 months	(35,135)	(3,722)
Estimated fair value, 12 months or longer	109,468	39,892
Unrealized losses, 12 months or longer	(6,154)	(3,719)
Estimated fair value, Total	1,562,369	423,919
Unrealized losses, Total	(41,289)	(7,441)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	52,796	27,219
Unrealized losses, Less than 12 months	(470)	(50)
Estimated fair value, 12 months or longer
Unrealized losses, 12 months or longer
Estimated fair value, Total	52,796	27,219
Unrealized losses, Total	(470)	(50)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	238,119	60,229
Unrealized losses, Less than 12 months	(9,970)	(557)
Estimated fair value, 12 months or longer	9,925	451
Unrealized losses, 12 months or longer	(293)	(49)
Estimated fair value, Total	248,044	60,680
Unrealized losses, Total	(10,263)	(606)
Debt Securities | Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	76,103	58,262
Unrealized losses, Less than 12 months	(597)	(183)
Estimated fair value, 12 months or longer	4,577	1,031
Unrealized losses, 12 months or longer	(52)	(14)
Estimated fair value, Total	80,680	59,293
Unrealized losses, Total	(649)	(197)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	221,531	60,882
Unrealized losses, Less than 12 months	(4,582)	(318)
Estimated fair value, 12 months or longer	135
Unrealized losses, 12 months or longer	(22)
Estimated fair value, Total	221,666	60,882
Unrealized losses, Total	(4,604)	(318)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	727,202	135,354
Unrealized losses, Less than 12 months	(16,564)	(889)
Estimated fair value, 12 months or longer	66,365	22,112
Unrealized losses, 12 months or longer	(1,762)	(246)
Estimated fair value, Total	793,567	157,466
Unrealized losses, Total	(18,326)	(1,135)
Debt Securities | Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months		6,544
Unrealized losses, Less than 12 months		(1,498)
Estimated fair value, 12 months or longer	20,303	15,302
Unrealized losses, 12 months or longer	(3,782)	(3,312)
Estimated fair value, Total	20,303	21,846
Unrealized losses, Total	(3,782)	(4,810)
Debt Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	137,150	35,537
Unrealized losses, Less than 12 months	(2,952)	(227)
Estimated fair value, 12 months or longer	8,163	996
Unrealized losses, 12 months or longer	(243)	(98)
Estimated fair value, Total	145,313	36,533
Unrealized losses, Total	$ (3,195)	$ (325)