Investments (Details 1) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale securities		$ 90.3	$ 13.5
Net gains on sales and maturities		0.0	0.0
Proceeds from Sale and Maturity of Marketable Securities	$ 0.0	90.3	13.5
Other-than-temporary impairment losses on marketable securities	$ 0.0	$ 4.7	4.7
Middlemount Mine [Member]
Schedule of Available-for-sale Securities [Line Items]
Valuation Allowances and Reserves, Balance			$ 52.3