Benefit Plans - Incentive Plans (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Long‑Term Incentive Plan
Incentive Plans
Performance period	3 years
Incentive Plan for Key Executives | Salaries and employee benefits
Incentive Plans
Incentive plan expense		$ 13.3	$ 12.7	$ 10.3
Long‑Term Incentive Plan
Incentive Plans
Performance period			3 years
Long‑Term Incentive Plan | Salaries and employee benefits
Incentive Plans
Incentive plan expense		$ 9.3	$ 5.6	$ 5.4