Borrowings - Narrative (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Percent of assets	30.00%
Maximum borrowing capacity	$ 1,890,000,000	$ 1,780,000,000
Percent outstanding advances	100.00%
Pledged as collateral	1,200,000	13,200,000
Borrowings	461,166,000	421,352,000
Weighted average interest rate	1.18%	1.16%
Interest rate, range minimum	0.47%
Interest rate, range maximum	3.81%
Unused funds	1,430,000,000
Percent of qualifying assets	95.00%
Mortgage Loans on Real Estate
Debt Instrument [Line Items]
Pledged as collateral	2,510,000,000	2,330,000,000
Securities Investment
Debt Instrument [Line Items]
Asset balance used to determine maximum borrowing capacity from federal reserve bank	1,800,000
Amount outstanding	0	0
Qualifying Loans
Debt Instrument [Line Items]
Asset balance used to determine maximum borrowing capacity from federal reserve bank	619,500,000
Putable
Debt Instrument [Line Items]
Borrowings	$ 51,200,000	$ 51,400,000