Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental cash flow information
Cash (paid) for interest, net of amount capitalized	$ (413.7)	$ (411.9)	$ (329.6)
Cash received (paid) for income taxes, net	5.2	39.7	(9.3)
Accounts payable related to construction costs	169.2	170.1	177.1
Increase (decrease) in restricted cash from the sale (purchase) of investments held in the rabbi trust	4.6	(59.2)	(60.2)
Amortization of deferred revenue	24.9	24.7	39.9
Restricted cash	19.7	33.6
Bostco
Supplemental cash flow information
Portion of Bostco real estate holdings sale financed with note receivable	7.0	0.0	0.0
AMP Trillium LLC
Supplemental cash flow information
Note receivable received related to the sale of AMP Trillium LLC	0.0	0.0	12.0
Capital assets received related to the sale of AMP Trillium LLC	$ 0.0	$ 0.0	$ 6.3
Equity method investment, ownership interest (as a percent)			30.00%

Future cash outflow to pay for purchases of fixed assets that have occurred.

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