Variable Interest Entities (Summary of Key Terms of NMTC Transactions) (Details) - Primary Beneficiary $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
NMTC 2
Variable Interest Entity [Line Items]
Loans to VIE	$ 37.7
Interest rate percentage	1.00%
NMTC 2 | Minimum
Variable Interest Entity [Line Items]
Interest rate percentage	0.71%
NMTC 2 | Maximum
Variable Interest Entity [Line Items]
Interest rate percentage	0.77%
NMTC 2 | US Bancorp
Variable Interest Entity [Line Items]
Amount to other entity	$ 17.5
NMTC 2 | Unicom
Variable Interest Entity [Line Items]
Amount to other entity	52.0
NMTC 3
Variable Interest Entity [Line Items]
Loans to VIE	$ 8.2
Interest rate percentage	1.00%
NMTC 3 | US Bancorp
Variable Interest Entity [Line Items]
Amount to other entity	$ 3.8
NMTC 3 | Unicom
Variable Interest Entity [Line Items]
Interest rate percentage	1.35%
Amount to other entity	$ 12.0
NMTC 4
Variable Interest Entity [Line Items]
Loans to VIE	$ 6.7
Interest rate percentage	1.00%
NMTC 4 | US Bancorp
Variable Interest Entity [Line Items]
Amount to other entity	$ 3.3
NMTC 4 | Unicom
Variable Interest Entity [Line Items]
Interest rate percentage	0.73%
Amount to other entity	$ 9.8
NMTC 5
Variable Interest Entity [Line Items]
Loans to VIE	$ 10.4
Interest rate percentage	1.00%
NMTC 5 | Minimum
Variable Interest Entity [Line Items]
Interest rate percentage	0.67%
NMTC 5 | Maximum
Variable Interest Entity [Line Items]
Interest rate percentage	1.24%
NMTC 5 | US Bancorp
Variable Interest Entity [Line Items]
Amount to other entity	$ 5.1
NMTC 5 | Unicom
Variable Interest Entity [Line Items]
Amount to other entity	$ 14.7