Investments (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012 investments	Mar. 31, 2011	Dec. 31, 2011
Investments [Abstract]
Investment securities, aggregate fair value, continuous unrealized losses of more than 12 months	$ 1,143,000
Investment securities, aggregate unrealized loss, continuous unrealized losses of more than 12 months	62,000
Number of investments in continuous unrealized loss position for more than twelve months	19
Unrealized Losses Related To Marketability Of Securities Or Issuers Ability To Honor Redemption Obligations	0	0	0
Investment value deemed to be OTTI	0
Investment securities, aggregate carrying value, pledged as collateral	$ 2,540,463,000	$ 2,604,467,000	$ 2,588,704,000