Credit Agreements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 689
Outstanding Unsecured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	114
Outstanding Secured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	474
Combined Credit Facility [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	300
Amount of unsecured borrowing capacity	100
Committed Secured Letter Of Credit Facility A [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 225
Maturity date of facility	Dec. 31, 2021
Committed Secured Letter Of Credit Facility B [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 80
Maturity date of facility	Dec. 31, 2018