New accounting standards ASU 2017-07 (Details 2) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans	$ (500,000)	$ 2,900,000
Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit)		1,800,000
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans		$ 3,100,000