Operating Revenues Natural Gas Utility Operating Revenues (Details) - Revenues from contracts with customers - Utility - Transferred over time - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 3,487.3	$ 3,617.8
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	399.0	405.1
Natural gas | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	382.9	390.6
Natural gas | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	261.7	264.3
Natural gas | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	121.2	126.3
Natural gas | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	13.6	13.4
Natural gas | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2.5	$ 1.1