Investments (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Held-to-maturity Securities		$ 0.0
Marketable securities	$ 0.0	4.7	$ 21.5
Equity Method Investment, Realized Gain (Loss) on Disposal	0.1
Equity method investments	4.7	65.3
Loss from equity affiliates	15.9	107.6	40.2
Other-than-temporary impairment losses on equity method investments	276.5		43.2
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue	160.0	165.0	157.0
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	31.7	27.8
Equity Method Investment, Summarized Financial Information, Noncurrent Assets	348.0	424.4
Equity Method Investment, Summarized Financial Information, Current Liabilities	362.2	382.9
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities	10.5	11.3
Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	0.0	58.0
Loss from equity affiliates	$ 7.0	98.5	33.5
Equity method investment, ownership percentage	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
Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	4.7	7.3
Loss from equity affiliates	$ 8.9	$ 9.1	6.7
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%	50.00%
Corporate and Other [Member]
Schedule of Equity Method Investments [Line Items]
Marketable securities		$ 4.7	21.5
Loss from equity affiliates	$ 15.9	107.6	$ 40.2
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	$ 65.2	347.2
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%
Middlemount Mine [Member] | Financing Receivable [Member]
Schedule of Equity Method Investments [Line Items]
Investments and Other Noncurrent Assets	$ 65.2	$ 319.6