Investments (Details 2) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Marketable securities			$ 0.0	$ 4.7	$ 4.7
Equity method investments	$ 0.5	$ 0.5	0.5	1.5
(Gain) loss from equity affiliates	(28.8)	26.2	(16.2)	15.9	107.6
Other-than-temporary impairment losses on equity method investments				276.5
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue			183.0	160.0	165.0
Change in deferred tax asset valuation allowance related to equity affiliates					(52.3)
Other-than-temporary impairment losses on equity method investments				46.6
Equity Method Investment, Summarized Financial Information, Current Assets	47.3	47.3	47.3	31.7
Equity Method Investment, Summarized Financial Information, Noncurrent Assets	263.4	263.4	263.4	348.0
Equity Method Investment, Summarized Financial Information, Current Liabilities	363.5	363.5	363.5	362.2
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities	50.3	50.3	50.3	10.5
Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 0.0	$ 0.0	0.0	0.0
(Gain) loss from equity affiliates			$ (22.6)	7.0	98.5
Equity method investment, ownership percentage	50.00%	50.00%	50.00%
Other-than-temporary impairment losses on equity method investments				276.5
Subordinated Borrowing, Name [Domain]
Schedule of Equity Method Investments [Line Items]
Other-than-temporary impairment losses on equity method investments			$ 229.9	229.9
Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 0.5	$ 0.5	0.5	1.5
(Gain) loss from equity affiliates			$ 6.4	8.9	9.1
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%	50.00%	50.00%
Corporate and Other [Member]
Schedule of Equity Method Investments [Line Items]
Marketable securities					4.7
(Gain) loss from equity affiliates			$ (16.2)	15.9	$ 107.6
Other-than-temporary impairment losses on equity method investments				276.5
Financing Receivable [Member]
Schedule of Equity Method Investments [Line Items]
Investments and Other Noncurrent Assets	$ 84.8	$ 84.8	$ 84.8	65.2
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%	50.00%	50.00%
Middlemount Mine [Member] | Financing Receivable [Member]
Schedule of Equity Method Investments [Line Items]
Investments and Other Noncurrent Assets	$ 84.8	$ 84.8	$ 84.8	$ 65.2