Debt and Equity Securities (Amortized Cost and Estimated Fair Value of Investments in Debt Securities, Classified as Available-For-Sale) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 18,816
Debt securities, Estimated fair value	2,825,477		2,819,817
Other-than-temporary impairments in AOCI	21,300		20,700
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,835,448		2,848,594
Gross unrealized, gains	21,391		18,371
Gross unrealized, losses	(31,362)		(47,148)
Debt securities, Estimated fair value	2,825,477		2,819,817
Other-than-temporary impairments in AOCI	21,292		20,743
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	66,542
Debt securities, Estimated fair value	66,804
U.S. Treasury Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	66,542		66,400
Gross unrealized, gains	708		669
Gross unrealized, losses	(446)		(685)
Debt securities, Estimated fair value	66,804		66,384
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	504,680
Debt securities, Estimated fair value	506,039
Municipal Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	504,680		491,143
Gross unrealized, gains	6,630		5,113
Gross unrealized, losses	(5,271)		(10,291)
Debt securities, Estimated fair value	506,039		485,965
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	189,005
Debt securities, Estimated fair value	190,346
Foreign Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	189,005		221,298
Gross unrealized, gains	1,527		1,836
Gross unrealized, losses	(186)		(626)
Debt securities, Estimated fair value	190,346		222,508
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	262,431
Debt securities, Estimated fair value	259,114
Governmental Agency Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	262,431		267,713
Gross unrealized, gains	558		233
Gross unrealized, losses	(3,875)		(5,401)
Debt securities, Estimated fair value	259,114		262,545
Governmental Agency Mortgage-Backed Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,457,000		1,426,489
Gross unrealized, gains	2,797		2,074
Gross unrealized, losses	(18,818)		(25,254)
Debt securities, Estimated fair value	1,440,979		1,403,309
Non-Agency Mortgage-Backed Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	18,816	[1]	19,658	[1]
Gross unrealized, gains	1,179	[1]	1,167	[1]
Gross unrealized, losses	(1,349)	[1]	(1,803)	[1]
Debt securities, Estimated fair value	18,646	[1]	19,022	[1]
Other-than-temporary impairments in AOCI	21,292	[1]	20,743	[1]
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	336,974
Debt securities, Estimated fair value	343,549
Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	336,974		355,893
Gross unrealized, gains	7,992		7,279
Gross unrealized, losses	(1,417)		(3,088)
Debt securities, Estimated fair value	$ 343,549		$ 360,084

[1] At March 31, 2014, the $18.8 million amortized cost is net of $0.5 million in other-than-temporary impairments determined to be credit related which have been recognized in earnings for the three months ended March 31, 2014. At March 31, 2014, the $1.3 million gross unrealized losses related to securities determined to be other-than-temporarily impaired. At December 31, 2013, the $1.8 million gross unrealized losses related to securities determined to be other-than-temporarily impaired. The $21.3 million and $20.7 million other-than-temporary impairments recorded in accumulated other comprehensive income (loss) (“AOCI”) at March 31, 2014 and December 31, 2013, respectively, represent the amount of other-than-temporary impairment losses recognized in AOCI which were not included in earnings due to the fact that the losses were not considered to be credit related. Other-than-temporary impairments were recognized in AOCI for non-agency mortgage-backed securities only.