Borrowings (Details 3) (Reverse repurchase agreements, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reverse repurchase agreements
Borrowings
Average balance during the year	$ 1,100,000	$ 1,100,000	$ 1,422,678
Maximum balance at any month end during the year	1,100,000	1,100,000	1,700,000
Balance outstanding at end of year	$ 1,100,000	$ 1,100,000	$ 1,100,000
Weighted average interest rate during the year (as a percent)	3.82%	4.06%	4.28%
Weighted average interest rate at end of year	3.62%	3.87%	4.32%