Schedule I - Condensed Financial Information of Registrant - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$ 307.5	$ 202.4	$ 209.9	$ 379.7
Operating assets and facilities, net	5,804.7	5,698.4
Investments in affiliated companies	387.0	348.5	357.7
Other assets	297.1	321.2
Total Assets	7,105.4	6,894.2	6,919.9
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	174.8	170.9
Debt	4,271.9	4,204.2
Other liabilities	222.1	220.6
Total Liabilities	5,758.2	5,614.0
Total Shareholders’ Equity	1,347.2	1,280.2	1,314.0
Total Liabilities and Shareholders’ Equity	7,105.4	6,894.2
Parent Company [Member]
Assets
Cash and cash equivalents	249.1	147.6	$ 162.3	$ 294.6
Operating assets and facilities, net	3,457.9	3,373.6
Investments in affiliated companies	2,179.7	2,255.3
Other assets	758.6	709.4
Total Assets	6,645.3	6,485.9
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	88.6	72.0
Debt	4,140.8	4,107.7
Other liabilities	1,068.7	1,026.0
Total Liabilities	5,298.1	5,205.7
Total Shareholders’ Equity	1,347.2	1,280.2
Total Liabilities and Shareholders’ Equity	$ 6,645.3	$ 6,485.9