Significant Accounting Polices - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Period of forbearance within twenty-four months (more than)	3 months
Period after grace period for forbearance allowance	9 months
Forbearance period after grace period (up to)	6 months
Percentage of loans granted forbearance qualified as TDR	69.00%		69.00%
Notional value	$ 5,900.0
Excess tax benefit, amount	7.3
Chicago Mercantile Exchange
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 5,100.0
Percent of total notional derivative contracts	85.80%
London Clearing House
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 700.0
Percent of total notional derivative contracts	11.90%
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