Capital (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Capital [Abstract]
Risk Based Capital Required	$ 781,940	$ 579,629
Risk Based Capital Actual	$ 5,146,274	$ 5,026,133
Regulatory Capital Ratio, Actual	4.72%	4.80%
Regulatory Capital, Required	$ 4,362,112	$ 4,185,411
Regulatory Capital, Actual	$ 5,146,274	$ 5,026,133
Leverage Ratio, Actual	7.08%	7.21%
Leverage Capital, Required	$ 5,452,640	$ 5,231,764
Leverage Capital, Actual	7,719,411	7,539,200
Retained Earnings, Appropriated	$ 290,644	$ 260,285