Employee Benefit Plans (Narrative) (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Estimated net loss from defined benefit plan, that will be amortized into periodic costs	$ 2,300,000
Retirement benefits based on years of service and compensation	5 years
Expected cash contribution	10,440,000
Termination liability	13,400,000
Increase in goodwill due to withdraw from Plan	8,200,000
Contribution plan, employer matching contribution percentage	50.00%
Defined contribution plans employee contribution percentage of eligible compensation matched by employer	6.00%
Shares allocated to the employee stock ownership plan	1.4	1.6	1.5
Contribution expense under employee stock ownership plan	3,400,000	3,200,000	4,300,000
Accumulated benefit obligation and projected benefit obligation	$ 48,700,000	$ 46,600,000
Funded status of Pentegra Plan, percentage	101.20%	84.40%
Equity Securities [Member]
Asses allocation target, current year, minimum	40.00%
Asses allocation target, current year, maximum	70.00%
Debt Securities [Member]
Asses allocation target, current year, minimum	30.00%
Asses allocation target, current year, maximum	60.00%
Cash and Cash Equivalents [Member]
Asses allocation target, current year, minimum	0.00%
Asses allocation target, current year, maximum	15.00%