Notes Payable and Long-Term Debt (Details) - USD ($)	9 Months Ended
	Sep. 30, 2015	Mar. 16, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Short-term Debt, Weighted Average Interest Rate	2.00%		0.80%
Long-term debt
Long-term Debt	$ 1,742,500,000		$ 738,800,000
Current portion of long-term debt	8,000,000		22,500,000
Long-term debt, net of current portion	$ 1,734,500,000		$ 716,300,000
Debt weighted average interest rate	4.60%		2.90%
Maximum borrowing capacity	$ 1,250,000,000
Estimated fair value of senior unsecured notes	1,697,900,000		$ 750,300,000
Fair value higher than carrying value for senior unsecured notes	48,500,000		98,700,000
Letters of Credit Outstanding, Amount	32,900,000		28,300,000
Short Term Borrowings [Member]
Short-term debt
Short-term borrowings	66,600,000		601,200,000
5.75% Senior Notes [Member]
Long-term debt
Long-term Debt	149,900,000		149,800,000
Debt instrument par value	$ 150,000,000
Debt instrument stated interest rate	5.75%
Debt instrument maturity date	Nov. 01, 2016
8.00% Senior Notes [Member]
Long-term debt
Long-term Debt	$ 134,000,000		134,000,000
Debt instrument par value	$ 134,000,000
Debt instrument stated interest rate	8.00%
Debt instrument maturity date	Oct. 01, 2019
4.625% Senior Notes [Member]
Long-term debt
Long-term Debt	$ 248,600,000		248,400,000
Debt instrument par value	$ 250,000,000
Debt instrument stated interest rate	4.625%
Debt instrument maturity date	Sep. 15, 2020
3.375% Senior Notes [Member]
Long-term debt
Long-term Debt	$ 499,000,000		0
Debt instrument par value	$ 500,000,000	$ 500,000,000
Debt instrument stated interest rate	3.375%
Debt instrument maturity date	Mar. 15, 2025
7.125% Senior Notes [Member]
Long-term debt
Long-term Debt	$ 119,500,000		119,400,000
Debt instrument par value	$ 121,000,000
Debt instrument stated interest rate	7.125%
Debt instrument maturity date	Feb. 15, 2029
4.375% Senior Notes [Member]
Long-term debt
Long-term Debt	$ 498,400,000		0
Debt instrument par value	$ 500,000,000	$ 500,000,000
Debt instrument stated interest rate	4.375%
Debt instrument maturity date	Mar. 15, 2045
Revolving Credit Facility [Member]
Long-term debt
Long-term Debt	$ 0
Current borrowing capacity	1,000,000,000
Term Loan Facilities And Other [Member]
Long-term debt
Long-term Debt	81,300,000		75,100,000
Unamortized portion of debt derivative [Member]
Long-term debt
Long-term Debt	11,800,000		12,100,000
Commercial Paper [Member]
Short-term debt
Short-term borrowings			$ 460,900,000
Long-term debt
Current borrowing capacity	$ 1,000,000,000.0