Fair Value Accounting - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Weighted average:
Loss severity	62.10%	49.60%
Default rate	2.00%	2.10%
Prepayment speed	13.50%	9.80%
Mortgage-backed Securities, Issued by Private Enterprises
Fair Value Inputs Quantitative Information [Line Items]
Aggregate overcollateralization	$ 27.0