Senior Notes - Narrative (Details) - USD ($)			12 Months Ended
	Nov. 03, 2017	Oct. 04, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 13, 2017
Debt Instrument [Line Items]
Principal value			$ 5,618,200,000	$ 3,318,200,000
Proceeds from issuance of long-term debt		$ 2,974,200,000
Payment for debt extinguishment			89,363,000	0	$ 0
Loss on extinguishment of debt	$ 12,600,000		12,641,000	0	$ 0
Write off of deferred debt issuance cost	$ 400,000
Maturities of Senior Notes
Aggregate maturities in 2018			8,000,000.0
Aggregate maturities in 2019			700,000,000
Aggregate maturities in 2020			1,011,200,000
Aggregate maturities in 2021			774,000,000
Aggregate maturities in 2022			750,000,000
Aggregate maturities in 2023 and thereafter			2,375,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 2,500,000,000.0
Floating Rate Notes due October 1, 2020
Debt Instrument [Line Items]
Principal value		500,000,000	500,000,000	0
2.50% Notes due October 1, 2020
Debt Instrument [Line Items]
Principal value		500,000,000	$ 500,000,000	0
Interest rate			2.50%
3.00% Notes due October 1, 2022
Debt Instrument [Line Items]
Principal value		750,000,000	$ 750,000,000	0
Interest rate			3.00%
3.90% Notes due October 1, 2027
Debt Instrument [Line Items]
Principal value		1,250,000,000	$ 1,250,000,000	0
Interest rate			3.90%
Senior Notes due 2018
Debt Instrument [Line Items]
Repurchased face amount		$ 700,000,000
5.15% Notes, due March 1, 2018
Debt Instrument [Line Items]
Principal value			$ 0	$ 200,000,000
Interest rate	5.15%			5.15%
Repurchased face amount	$ 200,000,000
Redemption price, percentage	101.252%
Payment for debt extinguishment	$ 2,500,000
6.50% Notes, due April 1, 2018
Debt Instrument [Line Items]
Principal value			$ 0	$ 500,000,000
Interest rate	6.50%			6.50%
Repurchased face amount	$ 500,000,000
Redemption price, percentage	101.941%
Payment for debt extinguishment	$ 9,700,000