Debt And Equity Securities (Amortized Cost And Estimated Fair Value Of Investments In Debt Securities, Classified As Available-For-Sale) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	$ 37,553,000		$ 42,100,000
Gross unrealized, losses	(7,800,000)		(11,900,000)
Debt securities, Estimated fair value	2,186,123,000		2,201,911,000
Unrealized losses for securities determined to be other-than-temporarily impaired	7,100,000		11,400,000
Other-than-temporary impairment, AOCI	29,700,000		32,100,000
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	2,153,913,000		2,167,580,000
Gross unrealized, gains	44,199,000		48,218,000
Gross unrealized, losses	(11,989,000)		(13,887,000)
Debt securities, Estimated fair value	2,186,123,000		2,201,911,000
Other-than-temporary impairments in AOCI	29,727,000		32,089,000
Other-than-temporary impairments	2,600,000		9,100,000
U.S. Treasury Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	109,035,000
Debt securities, Estimated fair value	110,062,000
U.S. Treasury Bonds [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	109,035,000		71,995,000
Gross unrealized, gains	1,843,000		2,236,000
Gross unrealized, losses	(816,000)
Debt securities, Estimated fair value	110,062,000		74,231,000
Municipal Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	357,372,000
Debt securities, Estimated fair value	374,711,000
Municipal Bonds [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	357,372,000		329,935,000
Gross unrealized, gains	18,013,000		19,272,000
Gross unrealized, losses	(674,000)		(84,000)
Debt securities, Estimated fair value	374,711,000		349,123,000
Foreign Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	224,883,000
Debt securities, Estimated fair value	226,415,000
Foreign Bonds [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	224,883,000		212,200,000
Gross unrealized, gains	2,186,000		3,026,000
Gross unrealized, losses	(654,000)		(206,000)
Debt securities, Estimated fair value	226,415,000		215,020,000
Governmental Agency Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	173,059,000
Debt securities, Estimated fair value	173,786,000
Governmental Agency Bonds [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	173,059,000		195,784,000
Gross unrealized, gains	1,100,000		1,970,000
Gross unrealized, losses	(373,000)		(1,000)
Debt securities, Estimated fair value	173,786,000		197,753,000
Governmental Agency Mortgage-Backed Securities [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	989,290,000		1,066,656,000
Gross unrealized, gains	9,207,000		10,816,000
Gross unrealized, losses	(1,143,000)		(925,000)
Debt securities, Estimated fair value	997,354,000		1,076,547,000
Non-Agency Mortgage-Backed Securities [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	37,553,000	[1]	42,089,000	[1]
Gross unrealized, gains	1,189,000	[1]	478,000	[1]
Gross unrealized, losses	(7,805,000)	[1]	(11,933,000)	[1]
Debt securities, Estimated fair value	30,937,000	[1]	30,634,000	[1]
Other-than-temporary impairments in AOCI	29,727,000	[1]	32,089,000	[1]
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	262,721,000
Debt securities, Estimated fair value	272,858,000
Corporate Debt Securities [Member] | Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized cost	262,721,000		248,921,000
Gross unrealized, gains	10,661,000		10,420,000
Gross unrealized, losses	(524,000)		(738,000)
Debt securities, Estimated fair value	272,858,000		258,603,000
OTTI Not Yet Recognized [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized losses for securities for which an other-than-temporary impairment has not been recognized	$ 700,000		$ 500,000

[1] At March 31, 2012, the $37.6 million amortized cost is net of $2.6 million in other-than-temporary impairments determined to be credit related which have been recognized in earnings for the three months ended March 31, 2012. At December 31, 2011, the $42.1 million amortized cost is net of $9.1 million in other-than-temporary impairments determined to be credit related which have been recognized in earnings for the year ended December 31, 2011. At March 31, 2012, the $7.8 million gross unrealized losses include $7.1 million of unrealized losses for securities determined to be other-than-temporarily impaired and $0.7 million of unrealized losses for securities for which an other-than-temporary impairment has not been recognized. At December 31, 2011, the $11.9 million gross unrealized losses include $11.4 million of unrealized losses for securities determined to be other-than-temporarily impaired and $0.5 million of unrealized losses for securities for which an other-than-temporary impairment has not been recognized. The $29.7 million and $32.1 million other-than-temporary impairments recorded in accumulated other comprehensive income (AOCI) through March 31, 2012 and December 31, 2011, respectively, represent the amount of other-than-temporary impairment losses recognized in AOCI which, starting January 1, 2009, 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.