Reconciliation of Total Revenue and Income Before Tax to Operating Revenue and Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	$ 2,987.6	$ 2,899.6
Net Realized Investment Gain (Loss)	1.1	(2.2)
Reconciliation of Income Before Income Tax to Adjusted Operating Income
Income Before Income Tax	353.3	341.1
Net Realized Investment Gain (Loss)	1.1	(2.2)
Operating Segments
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	2,987.6	2,899.6
Net Realized Investment Gain (Loss)	1.1	(2.2)
Adjusted Operating Revenue	2,986.5	2,901.8
Reconciliation of Income Before Income Tax to Adjusted Operating Income
Income Before Income Tax	353.3	341.1
Net Realized Investment Gain (Loss)	1.1	(2.2)
Adjusted Operating Income	$ 352.2	$ 343.3