Securities Sold Under Repurchase Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 637,100,000	$ 738,600,000
Short-term Debt, Average Outstanding Amount	535,500,000	959,500,000
Short-term Debt, Maximum Month-end Outstanding Amount	587,400,000	1,070,000,000
Assets Sold under Agreements to Repurchase, Carrying Amount	531,679,000	586,867,000
Assets Sold under Agreements to Repurchase, Interest Rate	3.55%	3.18%
Due within 1 yr
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	31,679,000	86,867,000
Assets Sold under Agreements to Repurchase, Interest Rate	0.05%	1.20%
Due within 2 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	2.47%	0.00%
Due within 3 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	0	50,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	0.00%	2.47%
Due within 4 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	400,000,000	0
Assets Sold under Agreements to Repurchase, Interest Rate	4.08%	0.00%
Due within 5 yrs
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	50,000,000	400,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	2.62%	4.08%
Thereafter
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	$ 0	$ 50,000,000
Assets Sold under Agreements to Repurchase, Interest Rate	0.00%	2.62%