Significant Accounting Polices - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Period of forbearance within twenty-four month period to be classified as TDR (more than)	3 months
Period after grace period for forbearance allowance	9 months
Forbearance period after grace period	6 months
Percentage of loans granted forbearance qualified as TDR	26.00%	26.00%
Notional value	$ 5,600.0
Benefit in income tax expense due to new standard	6.0
Chicago Mercantile Exchange
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 4,500.0
Percent of total notional derivative contracts	79.80%
London Clearing House
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 700.0
Percent of total notional derivative contracts	13.10%
Accounting Standards Update 2016-09 | Retained Earnings | New Accounting Pronouncement, Early Adoption, Effect
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cumulative effect adjustment to retained earnings	$ 0.2