Investments (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 investments	Mar. 31, 2012	Dec. 31, 2012
Investments [Abstract]
Investment securities, aggregate fair value, continuous unrealized losses of more than 12 months	$ 10,105,000
Investment securities, aggregate unrealized loss, continuous unrealized losses of more than 12 months	160,000
Number of investments in continuous unrealized loss position for more than twelve months	30
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	$ 2,558,079,000	$ 2,540,463,000	$ 2,351,072,000