Borrowings (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Borrowings
Amounts Outstanding as of End of Year		$ 6,200	$ 6,900
Securities Sold Under Agreements to Repurchase	1,925,998	1,901,084	1,600,000
Private institutions
Borrowings
Securities Sold Under Agreements to Repurchase	600,000	600,000	675,000
Funds Purchased
Borrowings
Amounts Outstanding as of End of Year	10,791	9,478	8,888
Average Amount Outstanding During Year	11,508	16,284	13,624
Maximum Amount Outstanding at Any Month End	10,791	77,832	33,888
Weighted Average Interest Rate During Year (as a percent)	0.17%	0.18%	0.16%
Weighted Average Interest Rate End of Year (as a percent)	0.14%	0.22%	0.21%
Securities Sold Under Agreements to Repurchase
Borrowings
Amounts Outstanding as of End of Year	1,325,998	1,301,084	943,717
Average Amount Outstanding During Year	1,245,763	1,052,558	581,998
Maximum Amount Outstanding at Any Month End	$ 1,369,378	$ 1,406,393	$ 1,124,794
Weighted Average Interest Rate During Year (as a percent)	0.08%	0.10%	0.31%
Weighted Average Interest Rate End of Year (as a percent)	0.07%	0.09%	0.15%