Schedule I - Condensed Financial Information of Registrant - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 202.4	$ 209.9	$ 379.7	$ 234.2
Operating assets and facilities, net	5,698.4	5,688.0
Investments in affiliated companies	348.5	357.7	354.3
Other assets	312.4	211.4
Total Assets	6,894.2	6,919.9	6,535.5
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	170.9	165.9
Debt	4,204.2	4,256.6
Other liabilities	220.6	246.1
Total Liabilities	5,614.0	5,605.9
Total Shareholders’ Equity	1,280.2	1,314.0	1,397.0
Total Liabilities and Shareholders’ Equity	6,894.2	6,919.9
Parent Company [Member]
Assets
Cash and cash equivalents	147.6	162.3	$ 294.6	$ 183.5
Operating assets and facilities, net	3,373.6	3,397.0
Investments in affiliated companies	2,255.3	2,199.5
Other assets	709.4	396.0
Total Assets	6,485.9	6,154.8
Liabilities and Shareholders’ Equity
Accounts payable and accrued expenses	72.0	54.9
Debt	4,107.7	3,821.1
Other liabilities	1,026.0	964.8
Total Liabilities	5,205.7	4,840.8
Total Shareholders’ Equity	1,280.2	1,314.0
Total Liabilities and Shareholders’ Equity	$ 6,485.9	$ 6,154.8