Debt and Equity Securities (Gross Unrealized Losses) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,377,139	$ 1,580,179
Unrealized losses, Less than 12 months	(22,909)	(41,734)
Estimated fair value, 12 months or longer	216,315	177,875
Unrealized losses, 12 months or longer	(9,102)	(8,174)
Estimated fair value, Total	1,593,454	1,758,054
Unrealized losses, Total	(32,011)	(49,908)
Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	33,207	85,112
Unrealized losses, Less than 12 months	(563)	(2,718)
Estimated fair value, 12 months or longer	1,554	1,046
Unrealized losses, 12 months or longer	(86)	(42)
Estimated fair value, Total	34,761	86,158
Unrealized losses, Total	(649)	(2,760)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,343,932	1,495,067
Unrealized losses, Less than 12 months	(22,346)	(39,016)
Estimated fair value, 12 months or longer	214,761	176,829
Unrealized losses, 12 months or longer	(9,016)	(8,132)
Estimated fair value, Total	1,558,693	1,671,896
Unrealized losses, Total	(31,362)	(47,148)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	31,282	37,492
Unrealized losses, Less than 12 months	(446)	(685)
Estimated fair value, 12 months or longer
Unrealized losses, 12 months or longer
Estimated fair value, Total	31,282	37,492
Unrealized losses, Total	(446)	(685)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	212,086	230,180
Unrealized losses, Less than 12 months	(4,081)	(8,938)
Estimated fair value, 12 months or longer	28,305	27,687
Unrealized losses, 12 months or longer	(1,190)	(1,353)
Estimated fair value, Total	240,391	257,867
Unrealized losses, Total	(5,271)	(10,291)
Debt Securities | Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	21,049	56,579
Unrealized losses, Less than 12 months	(186)	(626)
Estimated fair value, 12 months or longer
Unrealized losses, 12 months or longer
Estimated fair value, Total	21,049	56,579
Unrealized losses, Total	(186)	(626)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	165,253	203,011
Unrealized losses, Less than 12 months	(3,855)	(5,375)
Estimated fair value, 12 months or longer	136	131
Unrealized losses, 12 months or longer	(20)	(26)
Estimated fair value, Total	165,389	203,142
Unrealized losses, Total	(3,875)	(5,401)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	832,856	838,411
Unrealized losses, Less than 12 months	(12,852)	(20,970)
Estimated fair value, 12 months or longer	156,905	124,425
Unrealized losses, 12 months or longer	(5,966)	(4,284)
Estimated fair value, Total	989,761	962,836
Unrealized losses, Total	(18,818)	(25,254)
Debt Securities | Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months
Unrealized losses, Less than 12 months
Estimated fair value, 12 months or longer	11,805	12,086
Unrealized losses, 12 months or longer	(1,349)	(1,803)
Estimated fair value, Total	11,805	12,086
Unrealized losses, Total	(1,349)	(1,803)
Debt Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	81,406	129,394
Unrealized losses, Less than 12 months	(926)	(2,422)
Estimated fair value, 12 months or longer	17,610	12,500
Unrealized losses, 12 months or longer	(491)	(666)
Estimated fair value, Total	99,016	141,894
Unrealized losses, Total	$ (1,417)	$ (3,088)