Guarantees and Long Term Standby Purchase Commitments - Long Term Standby Purchase Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Long-Term Standby Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 3,100.0	$ 2,800.0
Weighted average remaining maturity, Loans underlying guarantees not held by transferor	15 years 1 month 10 days	14 years 7 months 10 days
Guarantee obligations issued after January 1, 2003 [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 4,900.0	$ 4,500.0
Guarantee obligations issued after January 1, 2003 [Member] | Long-Term Standby Purchase Commitments [Member]
Long-term Purchase Commitment [Line Items]
Guarantor Obligations, Current Carrying Value	31.8	30.3
Farm & Ranch Loans and USDA Guarantees [Member]
Long-term Purchase Commitment [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell	114.5	60.9
Rural Utilities Loans [Member]
Long-term Purchase Commitment [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell	$ 0.0	$ 4.0