Debt and Equity Securities (Gross Unrealized Losses on Investments in Debt and Equity Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,377,115	$ 970,289
Unrealized losses, Less than 12 months	(37,239)	(14,162)
Estimated fair value, 12 months or longer	262,575	481,237
Unrealized losses, 12 months or longer	(10,376)	(10,906)
Estimated fair value, Total	1,639,690	1,451,526
Unrealized losses, Total	(47,615)	(25,068)
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,143,885	761,367
Unrealized losses, Less than 12 months	(17,590)	(5,575)
Estimated fair value, 12 months or longer	246,587	478,897
Unrealized losses, 12 months or longer	(6,138)	(10,715)
Estimated fair value, Total	1,390,472	1,240,264
Unrealized losses, Total	(23,728)	(16,290)
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	44,043	8,122
Unrealized losses, Less than 12 months	(619)	(27)
Estimated fair value, 12 months or longer	1,667	15,124
Unrealized losses, 12 months or longer	(3)	(154)
Estimated fair value, Total	45,710	23,246
Unrealized losses, Total	(622)	(181)
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	117,512	137,755
Unrealized losses, Less than 12 months	(1,253)	(689)
Estimated fair value, 12 months or longer	13,980	19,625
Unrealized losses, 12 months or longer	(123)	(318)
Estimated fair value, Total	131,492	157,380
Unrealized losses, Total	(1,376)	(1,007)
Debt Securities | Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	26,589	6,215
Unrealized losses, Less than 12 months	(470)	(8)
Estimated fair value, Total	26,589	6,215
Unrealized losses, Total	(470)	(8)
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	42,938	27,479
Unrealized losses, Less than 12 months	(778)	(88)
Estimated fair value, 12 months or longer	18,589	127,936
Unrealized losses, 12 months or longer	(62)	(1,930)
Estimated fair value, Total	61,527	155,415
Unrealized losses, Total	(840)	(2,018)
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	559,912	383,717
Unrealized losses, Less than 12 months	(1,955)	(1,612)
Estimated fair value, 12 months or longer	189,496	300,918
Unrealized losses, 12 months or longer	(3,929)	(7,483)
Estimated fair value, Total	749,408	684,635
Unrealized losses, Total	(5,884)	(9,095)
Debt Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	352,891	198,079
Unrealized losses, Less than 12 months	(12,515)	(3,151)
Estimated fair value, 12 months or longer	22,855	9,683
Unrealized losses, 12 months or longer	(2,021)	(113)
Estimated fair value, Total	375,746	207,762
Unrealized losses, Total	(14,536)	(3,264)
Debt Securities | Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, 12 months or longer		5,611
Unrealized losses, 12 months or longer		(717)
Estimated fair value, Total		5,611
Unrealized losses, Total		(717)
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	233,230	208,922
Unrealized losses, Less than 12 months	(19,649)	(8,587)
Estimated fair value, 12 months or longer	15,988	2,340
Unrealized losses, 12 months or longer	(4,238)	(191)
Estimated fair value, Total	249,218	211,262
Unrealized losses, Total	$ (23,887)	$ (8,778)