Indebtedness (Activity) (Details) - USD ($)	1 Months Ended		12 Months Ended
	Nov. 30, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 30, 2017
Debt Instrument [Line Items]
Debt instrument, face amount			$ 2,921,050,000
Loss on debt extinguishment			49,000	$ 27,099,000	$ 0
Senior Notes [Member] | 3.25% Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate		3.25%
Debt instrument, face amount		$ 400,000,000
Senior Notes [Member] | 5.5% Senior Note [Member]
Debt Instrument [Line Items]
Stated interest rate		5.50%
Repurchased face amount of debt		$ 300,000,000
Loss on debt extinguishment		$ 3,800,000
Senior Notes [Member] | 6.625% Senior Note [Member]
Debt Instrument [Line Items]
Stated interest rate	6.625%
Repurchased face amount of debt	$ 296,500,000.0
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			2,300,000,000
Medium-term Notes [Member] | 7.5% Medium Term Note [Member]
Debt Instrument [Line Items]
Stated interest rate	7.50%
Repurchased face amount of debt	$ 100,000,000.0
Unsecured Debt [Member]
Debt Instrument [Line Items]
Loss on debt extinguishment	$ 23,600,000
Revolving Credit Facility [Member] | Credit Facility Group Lenders [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			800,000,000
Revolving Credit Facility [Member] | Credit Facility Group Lenders [Member] | 2014 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 800,000,000
Revolving Credit Facility [Member] | Credit Facility Group Lenders [Member] | 2017 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity						800,000,000
Commercial Paper [Member] | Credit Facility Group Lenders [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			100,000,000
Commercial Paper [Member] | Credit Facility Group Lenders [Member] | 2017 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity						100,000,000
Line of Credit [Member] | Working Capital Facility Lender [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 30,000,000
Line of Credit [Member] | Working Capital Facility Lender [Member] | 2017 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 30,000,000