Short-Term Notes Payable (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) Rate	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Credit facility, face amount		$ 400,000,000
Short-Term Notes Payable (Note 12)	$ 0		$ 952,000,000
Terms of Credit Facility	$ 150,000,000
minimum current ratio per credit facility | Rate	1.00
Interest coverage ratio | Rate	2.59
Current Maximum Leverage Ratio | Rate	2.75
Senior Secured Credit Facility April122016 Member [Member]
Short-term Debt [Line Items]
Credit facility, face amount			2,000,000,000
letters of credit sub limit	$ 750,000
Additional Capacity Available On Credit Lines At Request	$ 500,000,000
Minimum interest coverage ratio | Rate	2.50
Interest coverage ratio	3.49
Letters of credit outstanding	$ 325,676,000		258,177,000
Borrowings and issuance of letters of credit remaining capacity	$ 1,674,324,000		$ 789,823,000