Borrowings (Other Significant Details) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
DisclosureOfRepurchaseAgreementsAbstract
Average daily aggregate balance outstanding	$ 2,888,558	$ 3,417,892
Maximum outstanding balance at any month-end	$ 3,060,578	$ 3,466,450
Weighted average interest rate during the year	2.10%	2.74%
Weighted average interest rate at year end	1.77%	2.13%