Variable Interest Entities Unconsolidated VIEs (Details) - Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) entity	Dec. 31, 2016 USD ($) entity
Variable Interest Entity [Line Items]
Number of trusts | entity	3	3
Variable interest entity, reporting entity involvement, maximum loss exposure, amount | $	$ 62.1	$ 60.5