New accounting standards ASU 2017-07 (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans	$ 2.1	$ 1.9	$ 3.0	$ 6.9
Accounting Standards Update 2017-07 [Member] | Other Income [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit)		1.5		4.5
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans		$ 2.6		$ 7.9