Long-Term Debt (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012
Debt Instrument [Line Items]
Credit facility loan, maximum amount	$ 950.0
Borrowed unsecured credit facility maximum amount	$ 250.0
3.875% Convertible Senior Debentures, due November 1, 2029 [Member]
Debt Instrument [Line Items]
Interest rate	3.875%
Incremental principal amount of notes	$ 1,000
Conversion price per common share for convertible notes	$ 45.72
Conversion rate of shares per one thousand dollars of principal amount	21.8728
Contingent interest rate percent	0.375%
Trading period for contingent interest	5 days
Average trading price minimum per $1,000	$ 1,200
Common stock price as a percent of conversion price minimum	130.00%
Earliest period of conversion price	20 days
Consecutive trading days	10 days
Consecutive trading days	30 days
Trading price per debenture related to common stock	95.00%
3 3/4% Convertible Senior Subordinated Notes Due April 15, 2014 [Member]
Debt Instrument [Line Items]
Interest rate	3.75%	3.75%	3.75%	3.75%
Shares issuable upon potential conversion of convertible notes	4,567,330
Conversion price per common share for convertible notes	$ 21.37