Debt Line of Credit and Commercial Paper (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Unamortized Discount	$ 66,752
A 250 billion LOC [Member] [Domain]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	$ 2,500,000	$ 2,500,000
Debt Instrument, Maturity Date	Apr. 02, 2018
ability to increase LOC facility	$ 500,000
Debt Instrument, Basis Spread on Variable Rate	0.95%
Line Of Credit Facility Commitment Fee	15
Write off of Deferred Debt Issuance Cost	$ 400
Remaining borrowing capacity		2,070,000
Amount restricted/dedicated to support letters of credit		$ 45,100
Weighted average interest rate on revolving letter of credit		1.07%
200 billion LOC [Domain]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	$ 2,000,000
Debt Instrument, Maturity Date	Jan. 10, 2022
ability to increase LOC facility	$ 750,000
Debt Instrument, Basis Spread on Variable Rate		0.00%
Line Of Credit Facility Commitment Fee	12.5
Remaining borrowing capacity	$ 1,960,000
Amount restricted/dedicated to support letters of credit	20,600
Weighted average interest rate on revolving letter of credit		1.37%
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Commercial paper, maximum borrowing capacity	500,000
Commercial Paper, net	20,000	$ 387,300
commercial paper principal outstanding	$ 20,000	387,500
Unamortized Discount		$ 200
Weighted Average Interest Rate Commercial Paper	0.90%	0.56%
Commercial Paper Weighted average days outstanding	4 days	19 days
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | A 250 billion LOC [Member] [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 0.95%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | 200 billion LOC [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 0.825%