Securities Sold Under Repurchase Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 738,600,000	$ 1,280,000,000
Short-term Debt, Average Outstanding Amount	959,500,000	1,050,000,000
Short-term Debt, Maximum Month-end Outstanding Amount	1,070,000,000	1,060,000,000
Assets Sold under Agreements to Repurchase, Carrying Amount	586,867,000	1,055,763,000
Assets Sold under Agreements to Repurchase, Interest Rate	3.18%	3.64%
Due within 1 yr
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	86,867,000	205,763,000
Assets Sold under Agreements to Repurchase, Interest Rate	1.20%	3.08%
Due within 2 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	0	100,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	0.00%	300.00%
Due within 3 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	50,000,000	0
Assets Sold under Agreements to Repurchase, Interest Rate	247.00%	0.00%
Due within 4 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	0	150,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	0.00%	376.00%
Due within 5 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	400,000,000	100,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	408.00%	397.00%
Thereafter
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	$ 50,000,000	$ 500,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	261.60%	390.00%