Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Convertible Senior Notes Due 2018 [Member]	Dec. 31, 2012 Convertible Senior Notes Due 2018 [Member]	Dec. 31, 2013 ESSOP Debt [Member]	Dec. 31, 2012 ESSOP Debt [Member]	Dec. 31, 2008 ESSOP Debt [Member]	Dec. 31, 2013 Other miscellaneous debt [Member]	Dec. 31, 2012 Other miscellaneous debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				3.75%	3.75%
Number of common shares into which notes are convertible (in shares per one thousand dollar note)				64.3407
Face amount of note convertible into common shares				$ 1,000
Debt	569,200,000	572,900,000		550,000,000	550,000,000	18,000,000	20,800,000	30,000,000	1,200,000	2,000,000
Fair value	703,400,000	591,500,000		684,100,000	568,500,000	18,000,000	20,800,000		1,200,000	2,000,000
Debt, Weighted Average Interest Rate						3.69%	3.74%
2014	4,200,000
2015	3,300,000
2016	3,500,000
2017	3,900,000
2018	554,200,000
Interest Expense, Debt	$ 23,100,000	$ 34,600,000	$ 51,100,000
Maturity date of debt				2018	2018