Debt - Credit Facilities (Details)			3 Months Ended	6 Months Ended
	Jun. 29, 2018 USD ($) extension	Jul. 25, 2014 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 715,000,000	$ 715,000,000
Parent Company
Debt Instrument [Line Items]
Borrowings from credit facility				300,000,000
Repayments of borrowings				$ (300,000,000)
Weighted average interest rate outstanding borrowings			3.30%	3.20%
Debt weighted average maturity term			70 days	65 days
Maximum
Debt Instrument [Line Items]
Applicable margin on LIBOR loans	1.50%
Minimum
Debt Instrument [Line Items]
Applicable margin on LIBOR loans	0.50%
Revolving Credit Agreement
Debt Instrument [Line Items]
Line of credit facility term		5 years
Long-term line of credit			$ 0	$ 0	$ 0
Outstanding amount			0	0	$ 0
Revolving Credit Agreement | Maximum
Debt Instrument [Line Items]
Commitment fee percentage	0.25%
Revolving Credit Agreement | Prior To Amendment
Debt Instrument [Line Items]
Long-term line of credit		$ 4,000,000,000
Revolving Credit Agreement | After Amendment
Debt Instrument [Line Items]
Long-term line of credit	$ 3,000,000,000					$ 3,000,000,000
Maximum increase in borrowing capacity	$ 500,000,000
Debt covenants net leverage ratio	4.50%
Debt covenants net leverage ratio after year three	4.00%
Debt covenants net leverage ratio after year four	3.50%
Number of extensions | extension	2
Extended additional maturity period	1 year
Minimum indebtedness for other principal amounts	$ 100,000,000
Revolving Credit Agreement | After Amendment | Letter of Credit
Debt Instrument [Line Items]
Long-term line of credit	$ 1,000,000,000
Expected Maturity Date Year 2018
Debt Instrument [Line Items]
Outstanding amount			350,000,000	350,000,000
Expected Maturity Date Year 2019
Debt Instrument [Line Items]
Outstanding amount			$ 365,000,000	$ 365,000,000