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	Sep. 30, 2014		Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 3,096,600		$ 2,848,594
Gross unrealized, gains	27,352		18,371
Gross unrealized, losses	(23,068)		(47,148)
Debt securities, Estimated fair value	3,100,884		2,819,817
Other-than-temporary impairments in AOCI	19,877		20,743
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	70,909		66,400
Gross unrealized, gains	732		669
Gross unrealized, losses	(318)		(685)
Debt securities, Estimated fair value	71,323		66,384
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	534,865		491,143
Gross unrealized, gains	10,668		5,113
Gross unrealized, losses	(897)		(10,291)
Debt securities, Estimated fair value	544,636		485,965
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	199,882		221,298
Gross unrealized, gains	1,378		1,836
Gross unrealized, losses	(163)		(626)
Debt securities, Estimated fair value	201,097		222,508
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	204,546		267,713
Gross unrealized, gains	942		233
Gross unrealized, losses	(3,261)		(5,401)
Debt securities, Estimated fair value	202,227		262,545
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,522,943		1,426,489
Gross unrealized, gains	5,287		2,074
Gross unrealized, losses	(13,388)		(25,254)
Debt securities, Estimated fair value	1,514,842		1,403,309
Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	17,246	[1]	19,658	[1]
Gross unrealized, gains	1,362	[1]	1,167	[1]
Gross unrealized, losses	(1,057)	[1]	(1,803)	[1]
Debt securities, Estimated fair value	17,551	[1]	19,022	[1]
Other-than-temporary impairments in AOCI	19,877	[1]	20,743	[1]
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	546,209		355,893
Gross unrealized, gains	6,983		7,279
Gross unrealized, losses	(3,984)		(3,088)
Debt securities, Estimated fair value	$ 549,208		$ 360,084

[1] At September 30, 2014, the $17.2 million amortized cost is net of $1.0 million in other-than-temporary impairments determined to be credit related which have been recognized in earnings for the nine months ended September 30, 2014. At September 30, 2014 and December 31, 2013, the $1.1 million and $1.8 million, respectively, of gross unrealized losses related to securities determined to be other-than-temporarily impaired. The $19.9 million and $20.7 million other-than-temporary impairments recorded in accumulated other comprehensive income (loss) (“AOCI”) at September 30, 2014 and December 31, 2013, respectively, which relate to non-agency mortgage-backed securities, represent the amount of other-than-temporary impairment losses recognized in AOCI which were not included in earnings as the losses were not considered to be credit related.