Borrowings (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reverse repurchase agreements
Balance outstanding at end of year	$ 1,100,000	$ 1,700,000
Weighted average interest rate at end of year (as a percent)	3.03%	3.98%
Reverse repurchase agreements
Reverse repurchase agreements
Average balance during the year	1,422,678	1,926,575	2,275,068
Maximum balance at any month end during the year	1,700,000	2,100,000	2,500,000
Balance outstanding at end of year	$ 1,100,000	$ 1,700,000	$ 2,100,000
Weighted average interest rate during the year (as a percent)	4.28%	4.23%	4.14%
Weighted average interest rate at end of year (as a percent)	4.32%	4.30%	4.19%