Investments Other investment disclosures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Cost-method Investments [Line Items]
Amortization Method Qualified Affordable Housing Project Investments	$ 39.4	$ 34.2
Qualified Affordable Housing Project Investments, Commitment	$ 11.5	$ 11.3
Cost Method Investments, Additional Information	0	0
Income Tax Expense [Member]
Schedule of Cost-method Investments [Line Items]
Amortization Method Qualified Affordable Housing Project Investments, Amortization	$ 2.7	$ 1.6	$ 1.3
Affordable Housing Tax Credits and Other Tax Benefits, Amount	$ 2.2	$ 1.7	$ 1.3