Nonqualified defined benefit plan (Details 10) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans		$ 111.0	$ 105.2
Supplemental employee retirement plan, defined benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		1.8	7.1	$ 6.6
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	$ 3.3
Projected benefit obligation		101.8	110.8
Accumulated benefit obligation		$ 101.8	$ 104.6
Discount rate to determine benefit obligation		3.56%	3.77%
Rate of compensation increase to determine benefit obligation		0.00%	4.00%
Discount rate to determine net periodic benefit cost		3.77%	3.51%
Rate of compensation increase to determine net periodic benefit cost		4.00%	4.00%
Expected future benefit payments, next twelve months		$ 6.7
Expected future benefit payments, year two		7.1
Expected future benefit payments, year three		7.3
Expected future benefit payments, year four		7.7
Expected future benefit payments, year five		7.7
Expected future benefit payments, five fiscal years thereafter		$ 36.4