Significant Accounting Polices - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Period of forbearance within twenty-four months	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	27.00%		26.00%
Notional value	$ 5,800.0
Excess tax benefit, amount	6.5
Chicago Mercantile Exchange
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 4,600.0
Percent of total notional derivative contracts	80.50%
London Clearing House
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Notional value	$ 700.0
Percent of total notional derivative contracts	12.50%
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