Long-Term Debt (Details 3) £ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2015 USD ($)	Jul. 31, 2015 USD ($)	Aug. 29, 2013 USD ($)
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, Covenant Terms, Maximum Debt to Capitalization Ratio	The company also cannot permit the ratio of its debt to capitalization (as set forth in the Credit Agreements) to exceed 65 percent.
Line of Credit Facility, Covenant Compliance	the company was in compliance with all covenants under the Credit Agreements
Five Year Term [Member] [Domain] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,600,000,000	$ 1,800,000,000
Credit facilities	$ 0
Two Year Term [Member] [Domain]
Debt Instrument [Line Items]
Credit facilities	135,000,000		$ 0
Two Year Term [Member] [Domain] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 147,000,000	£ 120
Credit facilities | £		£ 110
Line of Credit Facility, Interest Rate Description	LIBOR plus 1.10 percent