Retirement and Other Employee Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plan | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 6.2	$ 5.9	$ 19.3	$ 17.5
Expected return on plan assets	(8.7)	(9.0)	(26.7)	(27.2)
Amortization of net loss (gain)	0.0	0.3	0.0	0.7
Settlement loss	0.0	0.0	0.0	0.0
Net periodic benefit cost	(2.5)	(2.8)	(7.4)	(9.0)
Pension Plan | Unfunded Nonqualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.8	0.7	2.2	2.1
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of net loss (gain)	0.2	0.4	0.8	1.2
Settlement loss	0.0	0.4	0.0	0.4
Net periodic benefit cost	1.0	1.5	3.0	3.7
Retirement Health Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.6	0.8	2.4	2.5
Expected return on plan assets	(0.5)	(0.5)	(1.4)	(1.6)
Amortization of net loss (gain)	(0.6)	0.0	(0.6)	0.0
Settlement loss	0.0	0.0	0.0	0.0
Net periodic benefit cost	$ (0.5)	$ 0.3	$ 0.4	$ 0.9