Borrowings (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010
Securities sold under repurchase agreements
Total contractual maturities of securities sold under repurchase agreements	$ 758,947,000	$ 1,925,998,000	$ 1,901,084,000
Securities sold under agreements to repurchase with private institutions
Securities sold under repurchase agreements
Total contractual maturities of securities sold under repurchase agreements	600,000,000	600,000,000	600,000,000
Total securities sold under agreements to repurchase placed with private institutions, at fixed interest rates	600,000,000
Weighted average interest rate for the company's agreements with private institutions (as a percent)	4.63%
Weighted average maturity period for securities sold under agreements to repurchase (in days or years)	4.94
Decreased weighted average maturity period for securities sold under agreements to repurchase is agreements terminated at earlier specified dates (in years)	2.51
Securities sold under agreements to repurchase with private institutions | Minimum
Securities sold under repurchase agreements
Remaining terms of these agreements with private institutions (in years)	3
Securities sold under agreements to repurchase with private institutions | Maximum
Securities sold under repurchase agreements
Remaining terms of these agreements with private institutions (in years)	10
Securities sold under agreements to repurchase with government entities
Securities sold under repurchase agreements
Weighted average maturity period for securities sold under agreements to repurchase (in days or years)	177
2 to 30 Days
Securities sold under repurchase agreements
Total contractual maturities of securities sold under repurchase agreements	71,000,000
31 to 90 Days
Securities sold under repurchase agreements
Total contractual maturities of securities sold under repurchase agreements	13,538,000
Over 90 Days
Securities sold under repurchase agreements
Total contractual maturities of securities sold under repurchase agreements	$ 674,409,000