Benefit Plans - Incentive Plans (Details) - USD ($) $ in Millions			12 Months Ended
	Jan. 01, 2017	Jan. 01, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long‑Term Incentive Plan
Incentive Plans
Performance period	3 years	3 years
Incentive Plan for Key Executives | Salaries and employee benefits
Incentive Plans
Incentive plan expense			$ 13.3	$ 13.3	$ 12.7
Long‑Term Incentive Plan
Incentive Plans
Performance period					3 years
Long‑Term Incentive Plan | Salaries and employee benefits
Incentive Plans
Incentive plan expense			$ 6.7	$ 9.3	$ 5.6