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	Jun. 30, 2014		Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,947,204		$ 2,819,817
Other-than-temporary impairments in AOCI	19,900		20,700
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,936,709		2,848,594
Gross unrealized, gains	29,639		18,371
Gross unrealized, losses	(19,144)		(47,148)
Debt securities, Estimated fair value	2,947,204		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	82,510
Debt securities, Estimated fair value	83,163
U.S. Treasury Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	82,510		66,400
Gross unrealized, gains	874		669
Gross unrealized, losses	(221)		(685)
Debt securities, Estimated fair value	83,163		66,384
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	516,201
Debt securities, Estimated fair value	523,535
Municipal Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	516,201		491,143
Gross unrealized, gains	9,362		5,113
Gross unrealized, losses	(2,028)		(10,291)
Debt securities, Estimated fair value	523,535		485,965
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	190,195
Debt securities, Estimated fair value	191,681
Foreign Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	190,195		221,298
Gross unrealized, gains	1,621		1,836
Gross unrealized, losses	(135)		(626)
Debt securities, Estimated fair value	191,681		222,508
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	225,555
Debt securities, Estimated fair value	223,892
Governmental Agency Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	225,555		267,713
Gross unrealized, gains	1,046		233
Gross unrealized, losses	(2,709)		(5,401)
Debt securities, Estimated fair value	223,892		262,545
Governmental Agency Mortgage-Backed Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,460,102		1,426,489
Gross unrealized, gains	5,956		2,074
Gross unrealized, losses	(12,001)		(25,254)
Debt securities, Estimated fair value	1,454,057		1,403,309
Non-Agency Mortgage-Backed Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	17,941	[1]	19,658	[1]
Gross unrealized, gains	1,346	[1]	1,167	[1]
Gross unrealized, losses	(1,293)	[1]	(1,803)	[1]
Debt securities, Estimated fair value	17,994	[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	444,205
Debt securities, Estimated fair value	452,882
Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	444,205		355,893
Gross unrealized, gains	9,434		7,279
Gross unrealized, losses	(757)		(3,088)
Debt securities, Estimated fair value	$ 452,882		$ 360,084

[1] At June 30, 2014, the $17.9 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 six months ended June 30, 2014. At June 30, 2014 and December 31, 2013, the $1.3 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 June 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.