Post Retirement Benefit Plans - Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Plan Assets:
Plan assets	$ 150.5	$ 131.4	$ 124.6
Group BPCE Life
Plan Assets:
Plan assets	0.5	0.5
Swiss Life Collective BVG Foundation
Plan Assets:
Plan assets	$ 150.0	130.9
Guaranteed minimum return on mandatory withdrawal portion of fund (as a percent)	1.00%
Guaranteed minimum return on non-mandatory withdrawal portion of fund (as a percent)	0.125%
The Company's percentage of returns above the guaranteed minimum	90.00%
Percentage of returns above the guaranteed minimum to be retained by Swiss Life	10.00%
Swiss Life Collective BVG Foundation: Bonds
Plan Assets:
Target allocation (as a percent)	65.00%
Swiss Life Collective BVG Foundation: Cash
Plan Assets:
Target allocation (as a percent)	2.50%
Swiss Life Collective BVG Foundation: Equity securities
Plan Assets:
Target allocation (as a percent)	7.50%
Swiss Life Collective BVG Foundation: Real estate and mortgages
Plan Assets:
Target allocation (as a percent)	25.00%
Significant Other Observable Inputs (Level 2)
Plan Assets:
Plan assets	$ 150.5	131.4
Significant Other Observable Inputs (Level 2) | Group BPCE Life
Plan Assets:
Plan assets	0.5	0.5
Significant Other Observable Inputs (Level 2) | Swiss Life Collective BVG Foundation
Plan Assets:
Plan assets	$ 150.0	$ 130.9