Nonqualified defined benefit plan (Details 11) - Underfunded Plan [Member] - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		$ 122,900,000	$ 111,000,000.0
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		68,300,000	62,500,000
Life insurance carried on plan participants [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		36,500,000	35,500,000
Other investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		18,100,000	13,000,000.0
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized		736,000	395,000	$ 207,000
Nonqualified Plan [Member] | Supplemental employee retirement plan, defined benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		4,700,000	1,800,000	$ 7,100,000
Curtailment gain	$ 3,300,000
Projected benefit obligation		102,500,000	101,800,000
Accumulated benefit obligation		$ 102,500,000	$ 101,800,000
Discount rate to determine benefit obligation		3.20%	3.56%
Rate of compensation increase to determine benefit obligation		0.00%
Discount rate to determine net periodic benefit cost		3.56%	3.77%
Rate of compensation increase to determine net periodic benefit cost			4.00%
Expected future benefit payments, next twelve months		$ 7,100,000
Expected future benefit payments, year two		7,300,000
Expected future benefit payments, year three		7,700,000
Expected future benefit payments, year four		7,700,000
Expected future benefit payments, year five		7,000,000.0
Expected future benefit payments, five fiscal years thereafter		$ 37,000,000.0