New accounting standards ASU 2017-07 (Details 2) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 1,400,000	$ 2,100,000	$ 900,000	$ 5,000,000.0
Accounting Standards Update 2017-07 [Member] | Other Income [Member] | Retrospective adjustment for adoption of 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit)		1,200,000		3,000,000.0
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans		$ 2,200,000		$ 5,300,000